If it's not too badly marked, I usually do what Antoni and Jay Jay have suggested, using the grittier white toothpaste and rub it all over with a cotton bud/Q-Tip, usually in circular motions, then rub/polish it off with clean cotton buds/Q-Tips, again, in circular motions. If that doesn't do it, then I sand the canopy with 4,000 / 6,000 and 12,000 grit paper, finishing with the toothpaste.
Cheers,
Martin : )