This Photoshop tip shows how to edit filters at a later stage using Convert to Smart Filters
Most filters are applied to a single layer, or sometimes to a duplicate, but they cannot be re-edited without starting over… or can they?
Use the Convert for Smart Filters command in the Filter menu (CS3 and onwards), and now any effect applied will be added as a kind of editable layer style.
By double-clicking on the bottom part of the layer (containing the name of the filter) you can re-edit the filter.
Filters can be stacked up too for a multi-layered effect. Once your image is converted to a Smart Filter, new filters added are contained inside this Smart layer.
You also have a handy mask attached to the Smart Filter to remove parts of the effect from your image.