Aside from the dry-fire mentioned already - which is a good idea to practice all the time to re-enforce the muscle memory, take a close look at your grip with the two guns side-by-side...
Using myself as an example - with a 92FS, I really have to be on my game to be "Accurate". I can still keep them all "vital", even at speed - and I am getting better, but I have a nasty pull down/left (7 O'clock) unless I take care to watch my trigger position...
Pick up a Sig226 and I can make nice tight groups in the center with my eyes closed (ok, slight exaggeration
)... Though I've fired the Sig in the past (prior to a long hiatus from guns), I've put 10X+ the number of rounds through the 92 vs the Sig at this point and the Sig is still night and day more accurate.
When I look at my grip on the two - the longer sweep of the Sig trigger and the shape of the grips puts me in just the right position. The 92 does not... So, no shocker that I have the consciously adjust my grip and position to compensate...
If your grip is wrong, you are using too many muscles in your hand and will pull one way or the other.
I've only had limited experience with the M&P, but have you changed the back-strap?
In my case, putting a bigger grip on the 92 would do the trick too, but would make it harder to carry... the M&P allows for easier adjustment, though it may not have enough range depending on what your geometry issues are...