The reason for ingrown hairs is that dead skin cells clogges up the passage for the newly shaved hairs to grow out. When the hairs starts to grow it can't get passed the layer of dead skin cells ant therefore starts growing under the skin. To remove these dead skin cells you thesefore need som kind of exfoliant so the hairs can grow freely.

As I understand it straight shaving also has a exfoliating effect and thats why, at least in my case, I haven't had any troubbles with ingrowns since I started with straight razors. I read somewhere that ingrowns are also more common with people who have curly beard, most common among black people, you might have a little curly beard..even if you aren't black. What you also can do is not to shave too closely..but then you won't get the smoothness of BBS.