This is an annoyingly common problem with houseplants. Fungus gnats are attracted to moisture, so the first thing to do is to make sure that you do not leave any runoff water in or under your plant containers and keep your surface soil on the dry side to discourage breeding. Your plants should be in a containers with drain holes so that they don't retain too much moisture. Going forward, watering plants from below will keep the surface dry but provide plants with necessary moisture.

If your plants are not too delicate, eradicate larvae from the soil by replacing the soil with a fresh sterilized mixture.  Watering the surface with a little castille soap mixed in water can also get rid of the gnat larvae - it usually takes a couple of applications to get rid of all of them.

Once you have corrected the conditions that allow fungus gnats to thrive and reproduce, you can get rid of remaining insects using yellow sticky trap available online. Natural controls take repetition and consistency but it is worth it to get rid of these annoying pests.

Courtesy of NYBG Plant Information Service