All the ointments and powders mentioned by others will help. However, I replaced every powder, ointment, and soap I carry with baking soda, the universal panecea. It also works well for chafing.
In addition, consider wearing boxer briefs (the best of both worlds), which pretty much prevent (ahem) everything from rubbing against anything. The Ex Officio brand works well for me.
I used to chafe like a sonofagun when I weighed 245, but the problem has pretty much disappeared since I lost 100 pounds. It's nice to see some space between my thighs. ;v) In other words, the best solution to chafing is to keep walking, never give up, and then walk some more.
Keeping the faith,