Not a stupid question in the slightest! I must admit I tried all sorts of kits, wipes and the lot - until I picked up a £5 screen cleaning kit from ASDA. Have to say it's genuinally been the best one I've found, got rid of a very nasty smudge / stain that nothing else really budged!
In case you haven't got an Asda where you are, it comes with a brush (to get rid of the loose bits of dirt first of all) then a spray and a cloth. 2 or 3 sprays on the screen, gentle wipe over with the cloth and dirt is gone