Not deet, bifenthrin. You spray it on your grass, bushes, rocks, etc. -- shaded places mosquitos hang out during the day. It kills them when they rest or try to feed (mosquitos are mostly vegetarians). It really does work and it's quite cheap ($35 for 3/4 gallon concentrate, which makes almost 100 gallons of spray).
Get a good sprayer; I use about 6 gallons to spray around my house every couple weeks (but it's a fairly large house with a lot of vegetation). The spray lasts longer in places where it doesn't rain so much.