In addition to Yamo's suggestions, it is sometimes helpful to simply look at the histogram and determine where the tonal levels are for significant objects and areas in your image. If there is too much overlap between adjacent objects/areas, B&W conversion will render the objects indistinguishable (or nearly so). In such cases, it may be desireable to work within the framework of the HSV color model to remap (i.e. selectively brighten/darken) objects or areas. This will make working with curves to enhance contrast a little simpler.
This is a simple method that almost always gives me good results
convert image to lab mode
click on the channels and then delete either the "a" or "b" channel
two channels now remain , delete the lower channel
convert back to gray scale and then rgb
play with levels and brightness/contrast