My first option? "Image>Duplicate"! Especially so if you are editing jpegs but any file really. I would also advise using Adjustment Layers rather than actually altering the image pixels. Much faster adjusting big files this way if you aren't running a hyped up computer and you can always cancel, edit etc w/out affecting the actual image pixels. Everyone has their own preference on procedures but I tend to start w/ a Curves adjustment layer and see what needs happening from there.
For difficult color balances (it happens) I am partial to using a Channel Mixer adjustment layer when simple Color Balancing doesn't solve everything. What else, oh save as a PSD file, at least as sort of a master file for important work. That way your Layers are saved, you don't run into re-compression loss each time you do an edit (which happens w/ jpegs) and you can always make dupes to save as smaller jpegs for posting, emails etc.
I'm only running CS not CS4 so I'm sure there are plenty of other cool things those Adobe phreaks have added but ideally you want flexibility w/out messing things up permanently. Some of the preceeding should help avoid that. // ww