There was a conversation on Twitter a while ago about catching fruit flies. I'd like to add my method (learned from a friend) to the pool of ideas.
Take a glass (jar, etc), put a piece of over ripe fruit in it. I used a banana, peaches work well too. Cover the top with plastic film (Saran Wrap, etc) and poke a few small holes in it with the tip of a knife. Small like 3/16".
Let it sit on the counter and wait.
Mine got so full you could hear them bouncing off the plastic wrap. You can even count them in the photo. Must have been 70 or so.