Question about preventing STI's

I want to be able to hook up with a guy, and even if he says he's clean (not infected) and I trust him, I'm always terrified that there's the small chance he's lying. On the other hand, I could just want a one night stand. So: how can you prevent sti's while having oral, vaginal or even anal sex? I know condoms don't completely protect you (and it's my suspicion that most people don't use condoms for blow jobs anyway...).