db,
flyinkiwi has hit the nail on the head:
Get into the habit of constantly asking yourself, am I in trim?, and doing something about it if you are not.
If you are having to apply any push or pull force on the yoke, you are not in trim. Adjust the trim wheel until there is no force and you can take your hand of the yoke, without the nose rising or falling.
If you briefly take your hand off every time you re-trim, your instructor will also know that you are in trim AND that you understand the concept. Re-trim every time you establish a new pitch attitude and/or power/flap setting, but don't forget to let the airspeed stabilize before trying to finalize your trim setting. This final adjustment can be very subtle - one or two degrees of rotation of the trim wheel.