Granted, it's not summer anymore, but I found this to be wonderfully effective a few weeks back when it was unseasonably warm. If only it worked on ants.....
All you have to do is put a small amount of apple cider vinegar into a jar, make a funnel out of a piece of paper and place it in the mouth of the jar (make sure the paper isn't in the vinegar) and you're done!
Within 5 mintues I had two fruit flies trapped in the jar. I actually think those were the only fruit flies in my house.
