A block of sal ammoniac is an excellent aid to keeping your soldering iron tip (or bit) clean and able to hold a small blob of solder.
A description of what sal ammoniac is and the safety precautions in its use are here.
You should place the block in such a way that it cannot slide around as you rub your iron over it.
Place your hot soldering iron tip on the block until it begins to smoke. Then move your iron slowly back and forth along the block. Initially, the block will be black from the contaminants coming from the soldering iron bit. As you rub the bit along the block, it will begin to clear. As it does, you can add a touch of solder and turn the bit over to check whether there are still any black spots on the face of the bit.
If there are still black spots, return to rubbing on the block for a time. If these spots are persistent, you can use a brass wire brush to help clean the contaminants off. Then add a touch of solder and return to rubbing along the block. Repeat this check until the whole bit is bright and holding a small blob of solder.
Repeat this process for the other side too.
Leave a small blob of solder on each side of the bit to protect the bit from oxidising. This cleaning process should be done at the end of each soldering session if the bit is not clean. But it does not substitute for the frequent wiping of the bit on a damp sponge to clean the bit as you work. The sal ammoniac block is for cleaning persistent contaminants off the bit.