Wipe your hot iron tip on a wet sponge on a regular basis while soldering. It must be a natural sponge, not a plastic based one. This should be a quick pass, rather than a lingering one to avoid cooling the tip of the soldering bolt. This keeps the tip clean of carbon and other contaminants that can reduce the effective heat from the tip.
If you have any dark gunk build up that won't come off on the sponge, rub the hot iron tip against a block of sal ammoniac until the block clears. If the dirt is difficult to remove with the sal ammoniac, use a brass wire brush to scrape the dirt off and then go back to the sal ammoniac block. When it is clean, add a touch of solder to re-tin the tip, and then wipe against your wet sponge.
Remember, all this is done while the iron is hot, so be careful.