Don't worry about it, it takes a bit of practice at first.  I havnt gone through the lessons (in fs9 i assume?) for a while, but what I have learned from real life (as well as endless hours behind my computer), its just practice practice practice. 

If you are maintaining altitude, make sure you fine tune your trim and get practice at trimming.  You should always be looking outside and glance at your instruments periodically to make sure your needle is pretty much dead on to your assigned altitude.  You should trim to the attitude outside, not to what your VSI says, and when controlling your aircraft, it should be as smooth as a feather. 

If your in cruise, maintaining your speed is pretty much is throttle work.  2300 RPM in FS9 (as well as in real life as a matter of fact), gives you around 90 or more knots.  If your climbing and descending, its a lot about controlling your attitude.  Pull back a touch to slow down, push forward to speed up while using your throttle to control your rate of descent.

Hope im helping