You don't need an expensive kit to clean an LCD screen. You can remove those smudges with common household items. And that goes for LED screens, as well, since they're simply LCD screens with a different type of backlighting.

You will need a microfiber cloth. If you wear glasses, your optometrist probably gave you one--although you might want a bigger one for a large screen. You can buy these for a few dollars from several Web sites, as well as from a photography store.

Use the dry cloth to gently wipe away the grime. If you encounter some stubborn dirt, apply gentle pressure; but not too much.

If that doesn't work, use a moist cloth. But how do you moisten it, and with what? You should definitely use soap, a window cleaner like Windex, or anything containing alcohol.