Well, like I said, I had a nasty fruit fly infestation this summer. I took the garbage out every night so any organic material wouldn’t hang around. Wine glasses, beer and cider bottles, and anything else that that might have had something sweet were rinsed thoroughly immediately. I poured a couple glugs of apple cider vinegar in empty bottles and cans with a squirt of dish soap and left them next to the sinks and cupboards. I hung pest strips everywhere they were hanging out and I sprayed Raid twice a day. A thorough spraying of the entire apartment just before I left for work in the morning and one room at a time in the evening where I’d give the space 15 minutes to air out before opening the door again.
I don’t know if it helped at all, but the internet told me that they like to lay eggs in drains so, every night, I’d boil a couple kettlesful of water and pour them down the sinks in the hopes they’d cook any eggs left behind.
In my fruit fly lab, we use cider vinegar + soap traps. They’re pretty effective and you can keep them out for several weeks (at some point there won’t be any volatile attractants left). A small dab of live yeast paste does an even better job at attracting flies, but you need to be diligent about discarding your traps every week so that you’re not breeding flies in even larger numbers. Put a funnel or inverted paper cone with a small hole in the bottom to catch more flies.
Don’t overlook a very low-tech solution: fly paper. You can buy it in drug, hardware, and feed stores. For those who don’t know, fly paper is a roll of paper covered in glue that you unroll and hang from your ceiling. It contains no pesticides.
To me, it is essential if you have a problem with any small flying insects. If you have a bad infestation, you will have to use as a supplement to the other methods, of course, and you will have to clean up whatever is attracting them. But it is awfully satisfying to wake up in the morning and see your fly paper covered with hundreds of the buggers.
Fruit (or drain) flies: from experience, get EVERYTHING off your countertops. Yes, including your microwave/toaster/mixer. Scrub everything with bleach water. Pay special attention to all the crevices around and behind your sink and faucet. They will breed in the tiniest bit of scum. In my case, I guess my counter was a little slanted, so water slopped from doing dishes would end up behind the nearby microwave. It wasn’t much, but there was a gross bit of biofilm back there, and that was the culprit. Little guys were gone after that.
Repeat as necessary.
Oh, and make sure you don’t have an old errant little potato/onion/apple somewhere around whatever bin you keep those things in; that’ll do it, too.