imo a better approach would be to have trim tabs available on the flight controls (perhaps just the elevators to start with) that could be moved in steps up and down with two mappable buttons. so then trim nose up and trim nose down keys with a display of where the trim is at similar to the vectored thrust display.

Farshid, this is not a good solution for us, fellow users of keyboard...

A good system that I have seen on simple flight Sims is a "Centre" feature which essentially freezes your trim setting when you activate the command. For example if you are consistently applying forward pressure on a Joystick to hold your pitch, when you press the "Centre" button the aircraft is perfectly trimmed requiring no more adjustments.

I have seen this on games like YS flight on the PC