A staple of the engineer's workshop, engineering files are capable of astonishing precision. A standard engineering apprenticeship task was to file a lump of metal into an accurate 1 inch cube, with measurably square sides and flat surfaces. Not an easy task, but it certainly teaches you the right way to use a file.
For best results, hold the file at both ends and at a slight angle to the work. Advance the file forward and across, with even pressure to bring the maximum number of teeth into contact with the work. Avoid rocking and lift off on the return stroke to avoid dulling the cutting ridges.
To clean your files gently brush out any accumulated debris with a soft wire brush or file card, always going along the ridges rather than across them. To minimise the chances of the file clogging when working with soft metals, apply chalk liberally to the surfaces before you begin.