What's a good bit with shoulder lift for a horse still in training. She has a light mouth but needs help with staying supple and lifting her shoulder. 

You can try a tender touch, it's soft yet will give you some lift. Another one to try would be the Brittany Pozzi lifter bit with a medium shank.

Maybe a pozzi lifter

What are you using now?

What are you currently doing with your body? (seat and legs?)  To keep the body in good position, and keep the shoulder up, you need to use your seat and legs to DRIVE them forward.

Yes, you want to select a bit that is going to help you and the horse the most, but remember that most of it comes from the seat/legs. Not the headgear.