Contrast/ Levels/ Curves
Works with all versions of Photoshop and Elements!
This is a new tip and an absolutely fantastic one - one that I use now exclusively!
Instead of using curves/ levels or brightness and contrast directly on your background layer, do the following:
1. Open up your image in PS
2. Make a copy of your background layer
3. Convert this copy to b+w. (There are numerous ways of doing it. Just have a search here in the Photoshop tips. I personally use Silver Efex Pro 2.
4. Dodge/burn/make level adjustments/ curves... on this black and white layer, until you are happy with your b+w version
5. Once done, set the blending mode to "luminosity" and change the layer opacity to your liking.
Voila done! What's good about it? You do not introduce colour shifts and you have two versions of your image: the colour one and the b+w one. (I am aware that you could use i.e. a level adjustment layer and then set its blending mode to luminosity, but I believe this method is more fun and faster)