I think the 3 best things you can do to protect your leather interior are:
1) Use a high quality leather conditioner (like the products at
www.colorplus.com)
2) Buy an $8 sun shade from Wal*Mart
3) Park in such a way to avoid direct sunlight (#2 can help with this).
I have the full leather interior, and it looks as good (or better) as the day I drove away from the dealership.