I’d start by investing in a few mosquito deterrents. You’ll need a good, reliable insect repellent that can be sprayed over your whole body, and it has to have DEET in it – not for the faint hearted but very effective. If you’re reluctant to smell like a chemical factory, try some of the natural solutions, like burning mosquito coils or placing citronella candles around your party perimeter (they give off a lemongrass type smell, so not too offensive). I’d also rig up some bug lights (bulbs with an opaque yellow finish). Mosquitoes are naturally attracted to lights, but if you coat the globe in an opaque yellow, then they become virtually invisible to insects (something about they see different wave lengths than humans).
If all of this isn’t enough, go for an automatic outdoor spray. These bad boys are designed to continuously douse your backyard in mosquito killer in the form of a fine mist – effective, but some guests may protest at the thought of your backyard being fogged over with insecticide.