The trick to eliminating these pests is to break their reproductive cycle. The most effective way is a two-pronged approach. First, get as many adults as you can by using sticky yellow cards or other trapping method. Second, and most important, is to kill the larvae. And where are those larvae? Why, they are almost undoubtedly living in your houseplant soil! I use a very effective Bacillus thurengiensis product called Knock-Out Gnats! I prefer this product to other Bt products which are traditionally used outdoors, because Knock-Out Gnats has very little odor. Some people use parasitic nematodes for killing the larvae, but I find that this method is too expensive for me.

Other methods to eliminate gnats are to remove any type of mulch you have in your potted plants, such as decorative moss. Some people report success by putting a thick layer of sand on top of their potted plant's soil, but this isn't very practical, can get expensive, and may not really work. Other folks dust the soil with diatomaceous earth (DE). This will probably work, as long as you don't water your plants. Once the plants are watered, the DE is less effective. However, in the short run this trick might be helpful. Make sure you don't breathe the DE dust, and don't let your pets breathe it, either. Something else to try is to put a bowl with a shallow amount of vinegar or wine near where the insects hang out. Usually they will fly to the liquid to investigate, and then drown. If you use a white bowl to hold the liquid, and place it under a light during the night (like a desk lamp or the light over your stovetop), you will probably be super grossed-out when you check it in the morning.

Other effective ways to control the population of these annoying creatures are: don't allow houseplant soil to stay constantly moist; eliminate overripe fruit & veggies (such as potatoes); make sure no vinegar or wine containers are open or leaking; and clean out & disinfect all your household drains, including the seldom used ones.

I don't really recommend using actual insecticides or pheromone traps indoors for this pest, because some people may develop sensitivies or allergic reactions. If you do resort to using insecticides , make sure your pets won't come in contact with the fumes, treated soil, or nibble on treated leaves. I've recently had a conversation with the folks at the Cornell Animal Poison Control Center, and they informed me that indoor pesticide use is a common way many pets are poisoned or killed. Please be careful!

FOR FURTHER READING:
http://ipmofalaska.homestead.com/files/fruitflies.html -
http://www.ext.vt.edu/departments/entomology/factsheets/fungusgn.html -
http://www.pestproducts.com/fungusgnat.htm -
http://www.daylilies.org/ahs_dictionary/fungus_gnats.html