The number one reason a student struggles in IMC is because they are not scanning properly.
If you can really make sure that they are scanning properly before they even touch an aircraft then your job will be a lot easier when you're in the real stuff. "Scan, scan, scan!".
Make sure they are not man handling the controls more then they have to. In something like a GPS arrival or an ILS approach a lot of students will try and fly the 3 degree profile by man handling the controls too much. They just need to set thier rate of descent and trim it out. I always ask them to take thier hands off the controls in these situations and 99% of the time the aircraft will pitch up or down quite a lot. If this happens then obviously its not trimmed properly and its demonstrating to them that they are fighting the controls.
Even in level flight most of them are making things worse by touching the controls more then they have to.
When changing power settings in IMC students will tend to focus on the RPM and MP gauges and will start to lose hdg, alt, pitch quite quickly. Comes back to scanning while doing anything.
And lastly, and this is for the benefit of the student but also for you. Really stress that they never handle the controls in such a fashion that they are creating slight g forces which start to screw with your physiological feelings and create illusions.
Most of the time when I say "check altitude" they will react unnecessarily by pulling back hard which creates a g force when they are only 200 feet below where they should be. 200 ft below assigned altitude isn't great but it doesn't deem a hard manouvre (unless perhaps in a critical situation like at the DA in an ILS). In a missed approach at night in ****e weather when they pull back harder then necessary it can really screw with your mind.