Sometimes you just have the type of hair that lends itself to getting follicle infections.
If you keep getting ingrown hair, you have to watch for it. Use a towel or a loofah to scrub at your skin after shaving.
Or try to wax regularly, as after a while you end up with finer hair that doesn't irritate the skin so much. It becomes finer too.
And there are some creams and things you can get which are supposed to soften the hair and your skin so that you don't get ingrown hairs.
If all else fails you can try IPL treatments. They helped me a fair bit although the treatment wasn't perfect.