Tack Cloth
A Tack Cloth removes loose dirt, hairs and so on from surfaces about to be painted.
They may be bought very cheaply from builders merchants, or are easy to make using a mixture of a little linseed oil, a few drops of turps & glycerine. The mix is stored in an airtight jar, and applied to damp lint free scrim to make a tack cloth.
Note this is only for use with oil-based paints. Using water-based paints (acrylic etc) a wipe down with a cloth dampened in clean water should suffice, and oil from a tack cloth may prevent the paint from adhering properly. For other sorts of paint such as cellulose the oils on a tack cloth may interfere with adhesion of the paint. Surfaces can possibly be wiped with a cloth dampened with the appropriate paint thinners.