After soldering, cleaning and drying your piece, use fine (000) or extra fine (0000) steel wool and a brass wire brush to go over all the beads. Use the brush first if you want to use it, and follow with the steel wool, since it will provide fewer scratches. This helps remove any leftover flux that on solder lines.
Omitting this step increases the chances of developing whitish blotches later on the solder lines. However, including this step makes a better base for applying the patina, if you are using any. After using the steel wool, use a soft brush to remove any residues from the steel wool, brush, flux or solder.