A lot of the ultra riders that I know use an acne cream with a drying agent in it. I personally don't use any creams because I feel it makes things worse in the long run. I will however use bodyglide in areas for long rides. It seems to breath well and prevents friction sores just fine for me.

But as already mentioned a good saddle fit and shorts fit is the best solution but often hard to accomplish. I find that it's really important to find shorts that fit up in your crotch area and don't move around. So although it seems counter intuitive, a too thick chamois will make chafing worse for me because the fabric gets bunched up and doesn't fit close in to my crotch.

It's also really important that your saddle height is correct and you're not rocking too much on the saddle.