Andy - if you're using PS 7, CS or CS2 there is a wonderful 'heal' tool near the top left of the toolbox it's just above the clone tool (which works well too) that is wonderful for blemishes, ketchup on faces, boogers in noses, etc.

My boys' pictures know that tool ALL too well. Set it on a soft brush and go to town. As opposed to the clone tool which simply copies a small portion of the picture and pastes it into the new spot, the heal tool tries to match textures and the surrounding colors. It works good, just takes a little practice. Use it in click click click fashion, not a click/drag fashion. Do one small change at a time so you can undo if one of the clicks doesn't look right.