Make sure she's clean. No cuts, bumps, etc. Any indication that she may do injection drugs is a automatic no-go.
Don't shave your genitals. It's best if she doesn't, either.
Ask if she's been tested, and when.
No BB anal, and avoid girls who offer it.
Take the drugs:
PrEP | HIV Basics | HIV/AIDS | CDC (start at least a week before fucking, and take it daily if you fuck often)
This section answers some of the most common questions about post exposure prophylaxis or PEP. You can also download PEP materials to share.
www.cdc.gov
If she's a reg, convince her to take them too.
People need to understand what their actual risk is.
Putting a number on it: The risk from an exposure to HIV. Fucking someone who is HIV+ gives you a 0.04% chance of contracting it. Taking precautions can lower it a lot. The risk isn't 0.. it'll never be 0 no matter what you do unless you're just jacking off.. but it's small enough not to worry.