What kind of icing do you use for decorating?
I use a royal icing for my cookie decorating. When it dries it's smooth and hard enough that the decorations keep their shape.
How do I get my number one tip to stop plugging up?
The tip plugs up when it is not in use and the air dries out your icing. When you put your icing bag down wrap the tip in a clean damp cloth and your icing will not dry up.