Here's a recipe for a mosquito repellant yard spray. My sister has used it at her house and said it works really well. Since I'm in the middle of a move, I haven't tried it yet.
1 cup lemon scented ammonia
1 cup lemon dishwashing liquid
25 gallon garden sprayer (like the Miracle Grow type that attaches to the garden hose)
Combine ingredients in the sprayer and spray the yard in early morning or in the late afternoon/evening. Do this 3 times each week and you should see the results quickly!