I've had some near my ass from teenage years. I'm not that big, but it happened anyway. I moisturized all the time. I don't think that really works. The marks aren't because your skin is dry, it's because it's being stretched. Some people get them easily, other's don't. Not much you can do without a cosmetic procedure.

At least every 6 months. Between free clinics, pop-up booths, HIV testing mobile units, home HIV test kits, and your general practitioner, it should be easy to get tested frequently and anonymously.