Grammy-winning mixing engineer Tony Maserati (Black Eyed Peas, Jason Mraz, Jay-Z, Beyoncé) demonstrates his approach to using the C4 Multiband Compressor on vocals. The information in this video also applies to Waves C6 Multiband Compressor plugin.
C4: http://www.waves.com/plugins/c4-multiband-compressor?utm_source=ytd&utm_medium=referral&utm_campaign=2s3yU14xpJ8#image
C6: http://www.waves.com/plugins/c6-multiband-compressor?utm_source=ytd&utm_medium=referral&utm_campaign=2s3yU14xpJ8#image