I use Gummi Pfledge.
I would remove the top, and place it on a stand or upside on a thick blanket. Then clean the seals with water (you could add some car wash if you wanted too) and use cheap towels because they will turn black. Then apply the Gummi Pfledge, and when you are done clean the applicator tip. I would then let it sit for about an hour before reinstalling the top. Gummi Pfledge is the best thing you can use on rubber in my opinion, it can actually help the seals plump back up.
