i found that putting 10m circles in a corner and as i exit the cirlce ask for canter, as this encourages the, to bend around your leg, also like sum1 said further up bending them to the outside helps, controls the shoulders from falling out, but make sure once they start to pick up the correct...
have you tried putting a peppermint cordial in their feed! it helped with our fussy horse, you can put as little or as much as you like!! or try treacle mis it with hot water so it thins up and then mix it in