I live in Nevada, and believe it or not, the cleanest safest girls are the licensed hookers. Here's how they stay disease free. (not saying its foolproof, but its the safest you can be)
When you're with someone new, ALWAYS take a shower together. Wash each others "area's" with antibacterial soap good. When you're naked, examine her pvssy closely. If you're skeched out about going down, either dont do it, use a dental dam (which I have never done, cuz it seems wierd), or just lick the outside areas and dont stick your tongue inside. It will still turn her on fiercely. Use a condom, obviously. When you're done, rinse your mouth with listerine, and take another shower, or at least wash your privates with antibacterial soap.
That's as safe as you can get, and thats how licensed hookers do it, and from what I understand, they hardly, if ever, catch anything.