Practice doing some 2-point. Do it first at a walk, and really sink your heels down. All your weight should be in your heels. Then move to a canter. Again, all of your weight ghouls be in your heels. Then, do two point for a couple strides, then sit for a couple strides, and repeat.
Get your trainer to give you a lunge lesson and canter on the lunge with no stirrups. Resist the urge to pull your knees up. Once you can do this well and keep your balance you can replicate it off the lunge with stirrups.