The traditional method for cleaning your soldering iron tip.  If the tip becomes dark and won't clean to silver with a swipe on a damp sponge, use salamoniac to clean it.  Simply heat your iron, rub it on the salamoniac block and then wipe it on your sponge.  When it gets clean, you need to re-tin it.  Melt a small puddle of solder on the block and rub it around with the tip.  The salamoniac will clean the tip and the solder will now stick perfectly.  This is an 8-ounce block.

The "modern" technique for cleaning your tip is to use Tip-Tinning Paste.  The SKU is HAKFS100.  Both methods work.  The Tip Tinning Paste is easier to use and non-abrasive which may make your tip last longer.