Although the more experienced houseplant growers are probably rolling their eyes at this, I get a lot of hits from people searching for ways to make their Spathiphyllum rebloom, so I figure I should probably try to address the question directly. It's slightly more interesting than it appears.
The first thing to realize is that there are a lot of varieties of Spathiphyllum, and some are more inclined to bloom than others.1 That's just the way it is. Larger varieties, like 'Mauna Loa' and 'Sensation,' are particularly unlikely to bloom, compared to the medium-sized and small plants.2
Second, the growers cheat. Most or all of the peace lilies available for retail sale have been forced to bloom, by application of a plant hormone called gibberellic acid.3 Gibberellic acid induces blooming roughly two to four months after it is applied, and is apparently reliable enough that plants can be sold in bloom at any time of year. Plants are extremely sensitive to gibberellic acid, and overapplication can harm the plants or produce weird, misshapen flowers, so it's probably not something you should be messing with. Also, as far as I know it's not easy to find at the retail level,4 so you likely wouldn't be able to use it even if you really wanted to.
Third, although a lot of people don't realize this because forced-bloom plants are available year round, peace lilies are seasonal bloomers, to a degree. The season begins in February or March, and ends around September. Some varieties will bloom sporadically in the off season,5 though it's unlikely and unreliable. So if you're in the wrong time of year for flowers, don't be upset if you don't see flowers.
Finally, we come to the things you actually can do something about.
My own plants will sometimes respond to a change in location by blooming. It doesn't seem to matter whether the new spot is better or worse than the old one: the mere fact of a move appears to be enough to get them excited. I don't know how solid the cause-effect relationship is, and I haven't seen anybody else recommend this, so the reader would do well to be skeptical, but it's something to try, and it won't cost you anything.
One might also try playing with the light levels: although Spathiphyllums will accept very low light, and can be bleached or even damaged by too much light,6 they do well if they're in a somewhat brighter spot than people usually give them. One of my plants, which I had not moved, once spontaneously flowered in mid-winter, because the angle of the sunlight coming in the (south) window finally got low enough to reach the plant on the other side of the room. It was weak light, and we didn't have sun every day, and the sun only lasted for an hour or two when it did happen, but getting that little extra hit of light intermittently seems to have given the plant enough of a push to form a flower. My 'Mauna Loa'/'Sensation' plant's threepeat blooming happened following a move to a location with multiple bright fluorescent lights. From everything I've seen, increasing light is the most reliable way to induce a bloom.
But what if you've got your plant in a bright spot already, and it's doing nothing? Peace lilies do best in a warm (65-90F; 18-32C), humid room: though they may live for a long time and look perfectly healthy and happy in a cooler7 or drier spot, it's not the ideal situation for the plant. Providing a more natural, tropical environment may be enough to convince an undecided plant to go ahead and bloom: try adding a humidifier or pebble trays and raise your thermostat.
I do not recommend changing your watering practices significantly. If you've figured out how to water a peace lily in a way that keeps it happy, you shouldn't mess with that, except insofar as giving your plant more light or warmer temperatures might leave it needing water slightly more often. (If, on the other hand, you haven't figured out how to water a peace lily in a way that keeps it happy, you should check out the PATSP profile for Spathiphyllum spp.)
One sometimes sees the claim on forums like Garden Web's that plants like Spathiphyllum and Hoya carnosa bloom better when potbound (i.e. when the roots have filled most of the volume in the pot). I think this confuses cause and effect. In my opinion (and it is only opinion, at this point: I don't have hard evidence to back it up), it's not that being potbound causes blooming, it's that age causes both blooming and potboundness. A younger plant, in both cases, is unlikely to bloom just because it's younger, not because it has room for its roots to spread out. There's no advantage in the wild to preferring a potbound state, since there aren't pots in the wild, so I don't see how there would be any direct advantage in the home either.
As far as it goes, I've had a plant or two bloom after a repot. I don't necessarily recommend repotting in order to get blooms, because I'm not convinced about the cause-and-effect issue there either, but that has been my experience once or twice. Potboundness, in any case, is unlikely to be the issue: peace lilies are capable of doing just fine in a cramped pot before complaining, and even then, your only observation is likely to be that they need watering a lot more often than they used to.
Your results w/r/t the whole potboundness issue may vary, and if they do, I'd be delighted to hear about them.
No, really. I would!
Fertilizer, like temperature and humidity, is unlikely to be the missing piece on its own, though if you have not fertilized or repotted your plant in a long time, it could be the case that missing nutrients might be holding it back. If you decide to fertilize your plant again after a long time without, DO NOT try to make up for lost time by adding more than the package recommends. That will only burn the roots and kill the plant. Just give the plant what the label says, or possibly slightly less than that. It might also be a good idea to deliberately pick a fertilizer formulation that contains micronutrients, specifically magnesium, as Spathiphyllums do have a higher magnesium requirement than most plants and consequently could run out sooner than other plants would. A good garden center should be able to direct you to an appropriate fertilizer. A bad garden center would -- wait a minute. Why are you shopping at a bad garden center?
Keep in mind that even if you do exactly what the plant wants you to do, flowers do not appear instantaneously, and it might still take a few months from the time you change your conditions to the time the buds start to show. So be patient.
To sum up, then. The best things to do to rebloom a peace lily are:
- Start with a variety that flowers abundantly to begin with,
- Move the plant to a brighter, warmer, more humid location,
- Between February and August,
- While delivering appropriate amounts of fertilizer,
- And apply a tiny amount of gibberellic acid to the leaves if you can find any.
1 Among the varieties I'm aware of: 'Lynise,' 'Sweet Chico,' 'Sweet Pablo,' 'White Flag,' 'Sensation,' 'Mauna Loa,' 'Domino,' 'Tasson,' 'Mauna Loa Supreme,' 'Deneve,' 'Ceres,' 'Figaro,' 'Viscount,' 'Starlight,' etc. A longer list, with descriptions for a few of the plants, can be found here, and a still-longer list without descriptions is here. If you don't know your plant's variety name, you may be able to rule out a few of these, but there's so much overlap and the differences between varieties are so subtle that you're unlikely to be able to definitively identify any particular plant by variety. Twyford International, a plant breeder/grower in Florida, says on their website that they produce ten Spathiphyllum varieties ('Avalon,' 'Claudia,' 'Double Take,' 'Emerald Beauty,' 'Florida Beauty,' 'Lynise,' 'Petite,' 'Sophia,' 'Starlight,' and 'Valentino,'), but then use the same paragraph of description for each one. See for yourself: go to this page and select "Spathiphyllum" from the first drop-down menu. My hope is that they're just being lazy, not that they can't tell the difference between varieties, but with peace lilies it could go either way. Oglesby is a little more helpful, providing a little bit of information about each of their nine Spathiphyllum varieties ('Milkyway,' 'Patrice,' 'Power Petite,' 'Prima,' 'Sensation,' 'Supreme,' 'Sweet Chico,' 'Sweet Dario,' 'Sweet Pablo'), and get downright gushy about 'Sensation,' though that still doesn't necessarily cover whatever variety you've got at home. My advice about Spathiphyllum cultivars: don't bother trying to figure out what you've got. You won't be able to, and the process will just make you angry. Even if you could figure out for sure what variety you have, figuring out whether it's a good bloomer or not is an additional layer of complexity, and information on that is even harder to come by. So just accept that you have a peace lily and don't know what variety it is. Trust me.
2 Though it's not impossible. I've had one since 2003 that has maybe flowered six times in the last six years. Trouble is, three of those happened one right after another during the fall and winter of 2008-09: most of the time, there's nothing. And for what it's worth, I find the big cultivars of Spathiphyllum a lot more easygoing than the smaller ones: less prone to turn black, slower to wilt when dry, etc. This is also the case with Dieffenbachia spp., that the larger varieties do better for me than the smaller ones. Your results may vary.
3 Flowering doesn't appear to be the usual response, in the plant kingdom, to gibberellic acid; it's main industrial application appears to be inducing seed germination and/or rapid growth. It just happens to trigger flowers in Spathiphyllum.
4 Though it's out there if you want to Google for it. I don't encourage this, frankly, because I don't especially like Spathiphyllum flowers in the first place, and the gibberellic acid formulations I found seemed really unreasonably overpriced, so the whole thing seems like a lot more trouble than it's worth, to me. But hey, if you care about the flowers that much, you know how to Google, don't you? (If I remember correctly, it involves putting your lips together and blowing. Though it's possible I have Googling confused with something else.)
5 (As, for example, did my big 'Mauna Loa' or 'Sensation' or whatever it is.)
6 Most peace lily cultivars should have dark green, glossy leaves. The exceptions are variegated types like 'Domino,' chartreuse ones like 'Golden Glow,' and there are a few gray-silver ones around too but I don't know any cultivar names for them. If your plant was dark green and turns lighter green, or yellowy-green, this is a sign of either too much light or a nutrient deficiency. Both are fairly easy to correct.
7 Something people don't know about spaths: they're surprisingly cold-tolerant. Where most tropical indoor plants will start to slow down or suffer damage at around 60F (16C), and a couple begin to have problems at the completely unreasonable temperature of 70F (21C), Spathiphyllums can go down to 40F (4C) and be fine.