After a ride, quickly get the area dry, this helps prevent them in the first place.

After a ride use some sort of salve or treatment. Good ones in this thread, I've also heard Bag Balm is good, and I've gotten good results from Aloe (for regular abrasion) and Cortisone cream (if it's a rash). Also, from the roadie forum, "Keith Bontrager says the pros use a mixture of a 13 oz vaseline container mixed with 1 oz neosporin, 1 oz lanolin (bag balm), and 1 oz cortisone cream." FWIW.

Because of where they're appearing, I'd check to see if your saddle is too wide, or you have a seam on your shorts.