One quick easy method is to change your buckling/strapping method. Most people buckle their boots up then wrap the power strap around the outside of the shell and snug it up. Instead, try this method: before buckling your top two buckles, wrap the power strap around just the liner and tongue, snuggly and push the strap as low as possible, then wrap the upper cuff flaps over the top of the power strap insuring that the medial side "D" ring is inside the plastic cuff, and buckle the top two buckles snuggly.
This method is used by many top pros and not only reduces fore/aft slop between the lower leg and boot top, it also stiffens the initial flex of the boot and offers a bit more control early in the flex range or at the top of the turns.
This costs nothing to try and makes a noticeable difference. If you desire more stiffness, then it is time to look at a higher performance model.