In 1998 I watched a Photographer friend of mine open a program called PhotoShop on his PC and proceed to re-touch a photo that he had just scanned.  Immediately, I thought to myself..  "It's impossible to take a bad photo now".  - I wasn't completely correct with that thought though.

Over the years I have learned the hard way to ALWAYS WORK IN LAYERS.

There are plenty of tutorials available on how to use Layer Masks and Adjustment Layers.  Get familiar with these and live by them! 

You should NEVER have to alter or edit your baseline photo - if you do, think of it as "damaging" your negative!

Hope this helps!