I have to admit I've spent WAY more time looking for rattlesnakes than I have geocaching, and there are a lot of great tips on here already, but thought I'd put my two bits in. If you are really really worried about being bitten, you can purchase a small, pack-ready vial of a genral purpose antivenin from some medical supply companies (look around on the web). The sooner you start receiving antivenom treatment the better off you will be. Just remember DONT cut the wound, this will only make it worse. DONT run- increased heartrate makes the venom work faster. DONT apply ice or a tight tourniquet, this often causes more tissue dammage than the bite itself. DONT try to identify the snake. The antivenom you will receive at the hospital is a "coctail" designed to work against the two types of venom you will encounter in the US. Snakes, even venemous ones, are beautiful and something to enjoy -at a distance.
Happy caching!