Tom Cosm offers his tips on using formant filters in Ableton Live:

This video is an answer to a question posted on my forums ( bit.ly/KyumG ) on how to get a good powerful sound out of your formant filter.

I have attempted to explain by relating the oscillator to the human vocal chords, and the filter as the mouth/nasal cavities in hope to show that having a solid sound/oscillation and pitch tonality pre-filter is the secret.