I made my own mouth protector (as many others have done) by just cutting apart an old paintball mask. After cutting a piece to fit over my lower face/mouth I just used some small bungie cords to secure it to my helmet. Others have used elastic straps instead that wrap around the entire head so they could wear their masks independently of a helmet. If I want to use it I just pull it down and over my mouth, if I don't want to I can stick it up on the front of my helmet.
Face protection > dental surgery!