I usuually use either a 'massarge' sponge as sold at Boots (one side is a rougher and bobbly sponge), or for more tricky marks, I gently use a nail brush and soap. Another alternative is the Flash magic sponges, these wear down quickly, but do work, just dont apply any more pressure than needed.
If the marks are on the fabric part, i just use the normal seat cleaner. Either Turtle Wax Interior Once (I think thats its name) or just been using the Autoglym one that comes in a kit with a good tough sponge, and 2 magic sponges, so may be the best one to get to clean all the interior. I also found it worked very well!