Few things (for me personally).
1. Certain level of acceptance that these things might happen sometimes. If I were, say, rock climbing, I might expect that I could be injured. I would take steps to prevent it, like using mats, chalking my hands, being a certified belayer, being supervised, being indoors, stretching first, etc...and then a moment of inattention might happen and I might break my elbow. FWIW, the recovery time for a broken elbow is a hell of a lot worse than the recovery time for any STI except HIV, and even then with modern medicine (and good health insurance) the stigma for having HIV is a hell of a lot worse than actually having it. The thing about most STI's is that...they really aren't that bad. Really. Listen to that podcast I linked above. It's just that because we've put the word "sex" in the front of them, people have a tendency to over-react. Like, this is not a "shit hit the fan" thing. A "shit hit the fan thing" is if I go in for my needle aspiration on Wednesday and they tell me I have thyroid cancer. A sense of perspective is important.
2. I fuck people who don't over-react to the possibility that I, or they, might contract an STI and then have to communicate about it. I have been part of a chain of people where a partner's partner contracted something, he may have been exposed, and I may have been exposed, and then I had to inform partners of mine that I might have been exposed. Everybody behaved like an adult in this four person chain, which is to say that the parties that were most closely linked to the possible source got tested, I did not shame or otherwise mistreat the person who may have exposed me, I waited to see what their test results were, and my partners did not shame me or otherwise mistreat me for my possible exposure. We did change our sexual behavior for several weeks to minimize their risk. If you cannot behave like an adult about these topics, don't fuck (not just multiple people, but anyone, ever -- "monogamous" people expose each other to STIs all the time). I have something of an advantage over some other people here, because the people I fuck know I have HSV and are thus typically more educated about these topics than your average bear.
3. Part of being responsible about being able to inform people is keeping a spreadsheet that tracks when I got tested, what I got tested for, who I fucked in the last six months, and how I fucked them. My partners have similar methods of tracking. One of them sends an email out to everyone he has fucked each quarter with his test results and an invitation to check in about the status of the relationship if you so desire (which is not to say that you can't talk to him whenever, of course, and I do, frequently).
4. Use of condoms with all partners. There's nobody in my life I bareback with for penetrative sex. I take certain risks with oral, it's true, and you can contract STI's orally (and I get anal, vaginal, and oral swabs as well as blood and urine tests. Many less informed people are not getting fully tested). You could also use dental dams or perform oral sex with condoms to increase safety. I also take daily valcyclovir to reduce the risks of passing on HSV to partners that may not have it. Some of my partners do not always use condoms with other people they have sex with. YMMV.
5. Regular STI tests for the most serious concerns, which (for me) are HIV, chlamydia, gonorrhea, syphilis, and Hep B/C. I don't bother with HSV testing because I know I have Type II, but a lot of doctors won't even test for it. If you ask for a standard panel, they will probably not test for HSV because the stigma is worse than the virus. I get checked about once a quarter. Some of my partners get tested that often, some slightly less often. For closed groups, or groups with more stringent agreements, you might all get together and decide how often all of you are going to get tested. At present I am sexually active with two people who are also sexually active with others; I don't insist on second-level out testing knowledge with my metamours. I trust that my partners are using good judgment on who they fuck and are taking care of themselves. YMMV.
6. I share results when I get them. So do my partners. Depending on your level of paranoia, you might insist on seeing actual paperwork from a doctor's office rather than just getting a text that says, "got tested for these things, negative for all." I've asked to see documentation in established relationships from people who don't insist on seeing mine. I've also just taken people at their word. People can also photoshop whatever they want, these days. YMMV.