How do you prevent std's when trying to conceive?

Uninfected partners. Have sex with someone who doesnt have an std. You should both be tested at the onset (and periodically thru your relationship) of sexually activity to establish baseline std risks. After that, stds are spread through infected genital secretions and skin-to-skin contact, and those things are needed to make babies, so there is always a risk if someone is infected.
Partner. You will need to be exposed to your partners bodily fluids if you want to conceive, your best protection then is to have your partner tested and treated if positive for any std.