"One of the most important steps in my conversion process is to use the “Histogram” to check the blacks and white and then to adjust them using “Levels” if necessary (it’s almost always necessary). After you have established a good black and white, you can use“Levels” to adjust the midtones and really change the mood of your image, for my images I generally am pushing the midtones darker.” Cole Thompson Photography Blog, Time No. 2—Before and After, http://www.photographyblackwhite.com/time-2/
Cole is talking about when converting from color to monochrome--I feel it is as important in color. Pull the white and black sliders in to kiss the upward slopes of the graph in the histogram and then adjust midtones darker.