^Yep, there's several other threads on martingales and their use on this forum.
In your case, without more information, I'd say that there shouldn't be a reason to ride your mare with a martingale. Instead, I would suggest checking for a physical reason for her head tossing (saddle fit, bridle/bit fit and adjustment, spinal alignment, etc.). If the usual suspects turn up clean, try some lateral flexion exercises and transitions. Be sure that your hands are soft and positioned correctly. Maybe look into an instructor/trainer to help you out. If it turns out that your horse does truly need a martingale, they can help you choose the right kind, adjust it, and use it correctly.