I have been using the leather conditioner from
www.colorplus.com with great success.
Other advice would be:
1) Don't clip stuff onto your belt- phone, Blackberry or etc.
2) If you wear a belt. make sure it is not rough (watch out for belts that your female passengers may wear!)
3) Watch out for metal rivets and buttons on certain pants and jackets
4) If you are wearing a belt try to have your shirt out over the belt to keep from rubbing on the leather