Teaching Pro and college coach who's been hitting a one-hander for a long, long time here. A few tips:
1) Eastern backhand grip. Don't use the continental if you're looking for more spin/loop and the ability to handle higher balls. Get that index-finger knuckle on top of the grip!
2) It's all in the shoulders. Once you start the forward motion of the swing, lock your torso in place and don't let the shoulders open up until after contact. I can't stress this enough. Once you've hit the ball, you can release. But for consistency with your contact, hold your shoulders steady.
3) Create a strong hitting platform with your legs. Remember the job of your legs isn't to directly add power to your groundstrokes. The job of your legs is, once you get to the all, to create a stable hitting platform that allow you to transfer power to the point of contact. Don't stand up in the middle of your shot, lift off, lean back, or otherwise shift your hitting stance unless you *have* to.