We've established that STD/STIs are not fun, and best avoided. Let us now look at how to do just that.
First off, as a nod to all the "just don't do it" folks, it is true that abstinence will greatly reduce your chances of contracting an STD/STI. (Reduce, though not eliminate entirely. While sexual contact is the primary source of infection, most of these can be passed in other ways.)
Condoms, condoms, condoms. We always hear about condoms, don't we. Well that is because, as a barrier method, they are extremely effective at preventing the spread of STD/STI that require exchange of fluids. Besides regular condom, the female condom and the dental dam are also barrier methods.
Condoms and dental dams won't 100% protect against any and every chance of infection. For example, herpes and genital warts are transmitted by skin to skin contact, and can be found in areas of the body not covered by the barrier. This doesn't mean that they are useless, but it does mean that you should do a little more than just use a condom and hope for the best.
A good step is talking to your prospective partner. The old adage of "you aren't just sleeping with them, but everyone else they've been with, and so on" is pretty accurate. Discuss possible risks. Get yourself and your partner tested, just to be sure. Be honest with yourself, and with your partner.
more later...