If you want to kill mosquitoes, your best bet is probably a bug zapper. If you just want to repel them, the Thermocells work really well. We have a Thermocell lantern. We usually put it out about 15 minutes or so before we are going to gather. The mosquitoes stay away. The little repellent pad has to be replaced every hour, but it does work well. They also sell personal thermocells that you wear.
If we go fishing and the mosquitoes are bad, I use an App on my smart phone that emits a high pitched squeal that keeps the bugs away. Adults can't hear it, but children sometimes can. It does keep mosquitoes away from me.
Flies are a pain. At our seasonal site, I set up disposable fly traps (not fly paper). The traps have a little pouch of attractant. Add water, the pouch with attractant dissolves and the attractant begins to attract the flies. Unfortunately, it takes a couple days to really start working. After about 4 or 5 days, it will remove all the flies in a decent sized area. The attractant begins to smell after a couple days and then really stink at about a week. The longest I've ever left one up is about 10 days. The bag is usually almost completely filled with dead flies. Just screw the top on and throw the bag away. At this point the bag stinks to high heaven, but the flies are gone.
At at our seasonal site, the flies don't seem to become a problem until mid July. So I usually buy a few of the bags at Lowes
. When the flies start to be a problem. I put a bag out. leave it for a week or so. Then after another week or so, I put another one out. They are very effective, but don't get too close to the bag after a couple days.
Bug Zapper will also work for flies, but it won't clear a large area like a fly trap will.