a few things to think about. are you breaking (bending) at the waist? If so you're probably going to turn towards the toe edge, keep that upper body straight. Try to shift your hips straight back (towards the tail) so that your back foot, hip, and shoulder make a straight line.
one other thing that sometimes help me smooth out presses is to think about "flattening" my back foot inside the boot...think about having your toes spread out and your foot flat, this will transfer to the snow.
Finally, try looking further ahead of you while you do your press, it helps...a lot. Go back to somewhere really gradual to get the press on lockdown before you go back to doing it on any kind of steeper terrain. Being on something mellow i.e. almost flat, will let you relax and experiment w/movements. just some things to try, hope it helps!