BMW sells a product called "Gummi-Pfledge" at the dealer. It comes in a small tube with a foam applicator built in. I'd use this product on the door seals. Using grease or wax could have an adverse effect on the rubber over an extended period of time. One tube will likely last you the rest of your life. Just squeeze and apply on all door and trunk rubber. Good for getting rid of squeaks too.

Regaards,
Troy
1995 E420 71k