I find a few things help me immensely in this area:

1) Body glide or petroleum jelly - I actually carry petroleum jelly based polysporin ointment
in my ditty bag, and so if I do have issues I ususally end up using that.

2) Good polypro underwear or tights with short legs - If I wear loose boxers or tighty whiteys
I find my bare thighs rubbing just under my groin get bad - if I wear nice supportive tights, or
polypro underwear (I usually wear the MEC polypro brand) that actually ends up with
materiel between there, I don't chafe up in the first place.

3) Keep everything clean. A good wash at the end of a hot day helps immensely - I carry
a few baby wipes to do this after a number 2.