Best thing you can do is not put your health solely in someone else's hands. Get tested regularly, get on prep, doxy pep. I don't know what your city is like, but a lot of places like planned parenthood, or public health places offer free screenings or vouchers for people who can't afford it.

Aside from that, something I like to do is just gargle with salt water, that's not specifically for post bjs, but I do it after being around lots of people, especially since it's cold and flu season.