A rasp and file cannot generally be sharpened and do not need setting up, however you can prolong their life by taking good care of them. Most of the time the sawdust which builds up in between the teeth will be pushed out by new wood fibres, however you can clear out the gullets by using a wire brush or a file card. If you do not have a file card, you can use an eraser to remove the fibres. Keep your woodworking files separate from your metalworking ones as they can become clogged with mixed fibres.
Tip: When storing rasps and files, do not store them with other metal objects as this can dull down the teeth with unnecessary friction.