Pour some bleach down all of your drains and if you have any inside plants, maybe check and clean the soil. Make sure no fruit is out and generally keep everything clean and put up and they will start to disappear !
You can seal off your home and make sure there are no holes in your window screens. You can also create a gnat trap by using empty bottles. Cut the top of the bottle and pour in some sweet juice. You can get more steps at http://www.thriftyfun.com/tf000909.tip.html.