Speaking as an ex competitive dressage rider (although nowhere near at that level!) please be assured Daisy the mouth frothing/foaming is just something that a lot of horses do, it's sort of acceptance of and playing with the bit, some racehorses do it too.
The neck flexion is also quite natural, look at film of wild horses fighting!
It is sort of a matter of energy control, the engine is at the back, you have to sort of stoke it up but be careful how you release it, think throttle & clutch?
Aaargh, trying to upload a picture of me & Woodstock warming up for a dressage competition at Windsor, but it just isn't happening|!