When they are really hard to get off I use my tool.
Since this picture I cut the bolts off and welded them to the handle. might be over kill for one bike but most new to me XS650's have these covers held on with some sort of sealant.
Side oil filter cover attached for the ages. hmm......... this worked well, two 1/2 bolts two 1/2 holes on 2 7/16" centers drilled in a suitable heavy chunk of bar. The bolts actually lightly thread into the recesses just enough for them to be nice and snug without creating any damage. keep the bar tight to the cover to reduce twisting on the bolts.
I must be more dumb than I thought. I don't get that one. And I have already tried using pliers (putting each side in the holes) and using a screwdriver/crowbar to twist it and nothing happens...
From this thread
http://www.xs650.com/forum/showthread.php?t=37