A damp sponge also works well for cleaning the tip. In case it isn't obvious, the cleaning should be done while the tip is hot.

And follow Matt's advice to wipe the tip on the damp rag or sponge between each and every soldering operation. It really helps. In other words, wipe the tip, solder a wire, wipe the tip, solder another wire, wipe the tip, solder a third wire, etc.

- Jeff