A dirty soldering iron tip is usually observed as being dull in colour and the solder drops off in small spheres as if the tip is repelling the solder.

A product for cleaning the soldering iron tip is available from either:

http://www.farnell.com
or
http://www.newark.com

This is usually available in a small pot about 1 inch diameter containing a solid chemical block. The hot iron tip is forced into the block which chemically re-activates it. However beware, the fumes given off aren't very pleasant.

Once cleaned, the solder tip should appear very shiny again and accumulate solder easily on the tip.

This product will only work for soldering iron tips which are iron-clad.

Iron clad tips are easily identified by the cladding which is usually very shiny like solder. The actual cladding, as the name suggests is a covering of the tip, thus making the tip slightly thicker.

Leaving the iron switched on for hours without being used will cause the iron cladding to become oxidized again and thus need re-activating. Just wiping the tip with a sponge isn't usually enough to clean it properly.