I'll give you an example. Someone asked me to adjust the lighting in a black & white photograph. There were a few things I noticed in the photograph:
With that information I came to the conclusion that it was a sunny day and that the shot was taken somewhere around noon. On cloudy days light is coming from all directions, since the sunlight bounces of the clouds. This creates soft dark shadows, but at the same time these shadows are not deep black because of the bouncing light that's coming from all directions which illuminates the shadows and makes them look lighter. In this case it was a shot taken in bright sunlight and this gives you an indication that the shadows need to be a deep black or in other words, that it's allowed to raise the contrast higher than you would do with a shot taken on a cloudy day.
Don't remove all red/orange colors that a candle is casting on the surroundings in a romantic shot. Removing it will damage the impact of the shot. Don't remove all color that a street light is casting on a sidewalk at midnight. Again, the reality is that the sidewalk underneath this street light will never look like pure gray. By removing all the street light colors you're abusing your imaging editor and you create an unrealistic scene and as a result you damage the great atmosphere of the shot.
Photoshop has many ways to work in a non-destructive way. Some examples of tools or adjustments that can be used in a non-destructive way:
Look at my my example that shows how such an approach might look like in Photoshop's layers palette.
I'm not going into details how all these non-destructive techniques work, but you can read more about it in my Non-Destructive Editing Photoshop tutorial. Also coinsider to have a look at the Beautify a Face Photoshop tutorial or the Retouch a landscape photograph Photoshop tutorial.
Being able to reconstruct an image in a non-destructive way doesn't only give you more control (=flexibility), but it also gives you a lot more pleasure.
Also make sure that you keep backups of the original files and in most cases it's best to keep a copy of the original on both your system and media like CD/DVD (re)writeables. My approach has always been:
Always be prepared for the worst. The higher rotation speed of modern 7,200 rpm hard drives also made them hotter and heat is what most drives can't handle over time. Add to this the fact that more than ever people have two drives inside their computer or use CPUs that create a lot more heat than years ago (either by design or because of overclocking) and you'll understand that heat has become a drive's worst enemy.
Click here if you want to read my article about Backup Strategies for Photoshop users.