Hi Julie,
Heather's method is great but you need another person and I never seem to have one handy when I want one!
My solution in that case with "planters" is to rock them sideways. If you can rock them off balance they are forced to move one foot and once they've moved one, more follow more easily. At that point you need to accept and praise movement in ANY direction, not necessarily the one you wanted

! Probably not for the faint hearted, but then the faint hearted wouldn't be doing this on their own I guess.
Hope it helps anyway.
C.