200-500 Hz is the frequency range responsible for warmth or body. Warmth is good but too much can make vocals boomy or mushy. Create instant clarity by removing between 200-500 Hz which will remove the muddiness.

If your singer sounds like a childish cartoon character, then cut 1-3 kHz. Don't go overboard though, or you will go from sounding like a cartoon character to having a serious head cold.

Presence is the key ingredient to intelligibility. If your vocals aren't standing out, try cutting 4kHz in guitars and other instruments. This will make room for vocals in the mix. Adding some 4kHz can add presence as well, but do it judiciously, since your vocals may end up harsh if you add too much.

Sibilance is the greatest vocal offender. "Esses" and other high-frequency sounds are obnoxious in speech and singing. While a de-esser is the best tool for the job, a prudent cut between 5 kHz and 8 kHz is often all it takes.