The careful application of a sharp knife and swearing when it eventually pings off and the knife hurtles towards your finger works best i find...
Normally, you can slip the knife blade under the foot and twist it slightly so they pop off, but some are glued down so hard you'll need to literally cut through the whole thing. Also, there seems to be two types of feet - plain feet and those with a moulded on circle - i find plain feet pop off easier (due to the smaller surace area) but with the circular feet i find it easier to remove the whole thing from the base and trim it down in increments rather then trying to cut them off while still attached to the base.