Hi ditto,
You're right. Trimming a plane in FS is a way different than trimming a real plane (I've never been in a plane but read a lot ;D

).
The trim in FS is made for non FF joysticks (or for keyboards). It adjusts the position of the elevator relative to your joysticks position such that you'll have your joystick in the center position when you trim the plane.
However in a real plane trim wheel never moves the elevators unless you let the stick free. It just adjusts the force on the elevator (thus the stick). So that you can adjust this forces such that you do not have to hold the stick when you settle to a level flight, climb or descent.
Coming to the solution of the problem, first, of course, you've to have an FF joystick. The point is that FF joysticks do not need to return to center position allways. However FS does not make use of this excellent property.
A few days ago I've come up with an add-on for FS by chance. That is FS Force. With this add-on you can have trim just as I described above for the real planes. I tried this and think it is very good. It also improves the feedback forces (return center forces change with speed, landing impacts, baking forces, etc. all adjustable).
The bad news is, this is an payware. Only 10 runs of FS :( . I'm now looking for an free alternative. Hope I'll find one. ::)