Condoms. The only way to minimize risk is to be in a truly monogamous relationship with a trusted partner who has been previously tested. If you have both been cleared then there is no risk (assuming no cheating). Condoms are effective in preventing stds passed via fluid transmission like gonorrhea, chlamydia & hiv. Condoms are less effective in skin to skin transmitted stds like hpv, herpes and syphilis.
Yes, if monogamous. If you're speaking of sex with multiple partners, or if you're likely to be having new partners from time to time, the answer is no: impossible. Condoms offer the best protection. However, you also can avoid STDs by having only a single uninfected partner and you both remain faithful to each other -- then no need for condoms.