I get them occasionally. I let them grow until they start digging in enough to start bleeding when I run or box, then deal with them.
I use a pair of cuticle scissors (I stole my pair from my wife) and cut straight back from the front of the nail parallel to the edge of the toe. This will hurt! Keep cutting unitl you can see some nailbed material welling up out of the cut, it looks somewhat like puss, but clearer and with an odd consistency, then grab the flap of nail with a pair of forceps (locking ones work the best) and simply rip the ingrown part out. That will hurt even worse!
Make sure you disinfect with alcohol (oh, yeah, that'll hurt, heh) and let it clot. I need to do it every four of five months on alternating toes, so a little over once a year per toe. I could go to a professional and probably have it taken care of permanently, but I enjoy the squirms it gives my wife, and while it does hurt, it's not crippling.