I use a great substance called "water." FWIW, it's what Apple recommends: http://support.apple.com/kb/HT1307

For really dirty screens, I use Windex on a soft cloth (spray the cloth, not the screen). There's some odd paranoia about not using Windex--"It'll etch the plastic!" or "You'll remove the antiglare coatings!"--but I've been using it for decades on computer screens with no problems.