+1 to other cleaning methods, but sanding a soldering iron tip is very bad. Any tip worth paying for has a copper core with a protective plating on it. That plating is usually iron or iron alloy. Sanding will remove the plating very quickly and expose the copper beneath. Other cleaning methods should remove only the microscopic oxidation layer, doing much less damage to the tip.
Copper has great thermal conductivity, but it dissolves in molten solder. So sanding the tip will make it work great for a while, but the exposed copper will dissolve quickly, rendering the tip useless. One of the failure modes of tips is that a tiny hole develops in the plating, and the copper dissolves from the inside. I once had a tip that wouldn't transfer heat any more. Eventually the very end broke off and I discovered that the core was filled with solder and only the iron plating had been keeping the shape of the tip. Now when a tip quits working well, I chuck it and get a new one. For a few bucks it isn't worth screwing around with a bad tip.
By the way, iron has good resistance to soldering environment, but the thermal conductivity sucks, which is why the plating is kept thin. Tips with thick plating wouldn't work well.