Basic Black and White Printing Corrections:

Grayish prints (often termed "flat", or "weak"):
Increase contrast by using a higher filter number.

Hard, contrasty, "soot and chalk" prints:
Decrease contrast by using a lower numbered filter.
NOTE: If your negative has been underexposed, you won't be able to lower the contrast without making the print look flat and gray. Underexposed negatives are a lose-lose situation!

Print too dark overall (both highlights and shadows):
Reduce the print exposure time and re-evaluate.

Print too light overall (both highlights and shadows):
Increase the print exposure time and re-evaluate.

Highlights dark, shadows OK:
Increase contrast by using a higher filter number.

Highlights too light and "blown out", shadows OK:
Decrease contrast by using a lower numbered filter.

Shadows too dark, highlights OK:
Decrease contrast by using a lower numbered filter.
NOTE: If the negative has been underexposed (thin), then the shadows will just get gray instead of becoming rich and luminous. Avoid underexposed negatives! With negative film you've got plenty of exposure latitude on the overexposure side - use it when in doubt!

Shadows too light and gray, highlights OK:
Increase contrast by using a higher filter number.

Also...
Be sure to evaluate corners and excessively light or dark areas in the picture as
candidates for dodging and burning.
 

Other problems:

• Mottled or "swirl" appearance and weak blacks in print - insufficient time and/or agitation in developer, or developer depleted.
• Yellow or brown stains on print that appear days later - fixer not completely washed out.
• Tinted (purple or pink) stains on print visible in minutes or hours - fixer bad or not fixed long enough.
• Dark center in print, negative OK - condensers not placed correctly.
• Print not sharp - if grain of negative visible, problem is with the negative. If grain is not visible, enlarger needs focused. Use grain magnifier.
• Print (and grain) sharp on one edge, not on another - enlarger needs aligned or negative not seated properly in enlarger. As a temporary workaround, try stopping down the enlarger lens or move to another enlarger.
• Black areas in print "smeared" out into lighter areas, whites fogged - dirty enlarger lens and/or enlarger lens flare.