Quote - ... make colour/levels/exposure changes via 'adjustment layers' (Top menu/Layer/NewAdjustmentLayer etc). These sit on the top level of whatever layers you have in the document. Save as a PSD.

You can continue making changes for as long as you like.

You can also use adjustment layers in combination with layer masks for more control.

Yup, I'm a big fan of this method because of its 'non destructive' nature (ie: the original image isn't permanently altered, it just looks different because of the adjustment layers above it) and its flexibility - the fact that you can go back and re-tweak the settings (including opacity) in any adjustment layer at any time.

You can also drag a stack of adjustment layers from one image file to another (like from a torso tex to a head tex, for example) and they'll have the same effect on the new file.

A useful tip: if you make a selection before you create the adjustment layer, the adjustment will be automatically masked to include/exclude your selection (alt key determines which - I can never remember). So you can have an adjustment layer which gives vicky a tan but doesn't include bikini-line areas or fingernails because you selected those areas first.

An adjustment layer usually affects all layers below it, but it can be made to affect only the first layer below it by alt-clicking on the line between the adjustment layer name and the next layer name down (in the layer name list). This feature can be really handy.