step 1 for me is use a pressure washer to clean first.
(just be careful of the decals...they may get blasted off)

You'd be amazed how much cleaner the top becomes just by pressure washing it....especially white tops.

If you don't have a pressure washer just go to your local coin operated car wash...the kind with the pressure washer "wand" in the booth.... bring a stepladder so you can get above the roof-line....balance yourself carefully, those wands can have a nasty kickback...