This works I always have fruit out and thats where they come from and also the heat.Ok take a plastic bottle cut the top off and turn it upside down into the part you cut. Mix cider vinager and suger. IT works every time. The nats are attracted to the mixtue they get in there and can;t get out. I have one that I keep on my kitchen counter in the warm months
Here you go!
This works...i've used it many times over the years. I think those little critters come in with the fruit I bring home and hatch out...I HATE them!
Good Luck! here's the link: http://www.ehow.com/how_2153551_rid-gnats-fruit-flies.html