I had a sixteen year old gelding that was very heavy on the bit, wouldn't stop, tossed his head, etc. I put him in a smooth mouth snaffle and started doing some flexing work (lateral, and then vertical). It took him a while to learn how to slow down (needless to say we loped a lot of circles to keep him from running away in a bit he didn't respect), but eventually he started to figure it out. I put him in a bit with shanks the other day and it is a night and day difference. Putting an older horse in a snaffle worked great for me!