If I get any finger marks on my iMac's screen I find the best way to clean them is by doing the following.
Get two micro fibre cloths, dampen one and wring out the water so this its just damp and not dripping wet.
Wipe the iMac's screen with this to remove the marks.
Next, get the dry micro fibre cloth and wipe the screen dry.
I find this method works just as good (if not better) than the overpriced screen cleaning kits.
I also have a large camera lens cleaning cloth that I use to wipe the screen if it isn't that dirty. This also cleans the screen extremely well too.