After the entire working surface of the iron tip has been coated with solder, clean off any excess solder with your brass or damp cellulose sponge.

"Stabbing" the brass sponge or wiping the tip with a damp cellulose sponge a few times should do the job.

Whether you're actively soldering or about to store your iron, you should always keep your tip tinned for these two reasons:

A tinned tip transfers heat much more effectively, making soldering a breeze.

The coat of solder creates a barrier between the tip and environment, dramatically reducing the rate of oxidation.