Talcum powder, or cornstarch, will eventually absorb moisture (and odor) and clump. This will cause more irritation. Powders are grains etc., equals increased friction.
The recommendations above are awesome-just my 2 cents here though. Diaper rash cream may work (Zinc oxide). (Try to avoid petroleum based products), You will need to wash it off well at the end of the day. you may need to apply it several times during the day as well.
Try boxer briefs (cotton/silk). You may need too change these throughout the day. Losing adipose tissue may help(If this is the problem. I understand some people just have large thighs due to genetics, and that it may not be fat.).
What are you wearing over top of this area? If you are wearing ie denim, which has several friction points and no breathability, and if they are too tight it won't matter what you wear underneath. It will cause friction, increase moisture and humidty-you don't want that.
Soaking the area with Epsom salts will help (hurts like heck, but works).
Try looking up Woundcare Journals on the Net. Hope this helps.