Fred R
Here's a tip - presume you are flying something with a trim wheel in the instrument panel? eg PA28, C1502/72 If so..
Imagine your hand is one top of wheel (which it can almost be). If you push the wheel AWAY from you it is as if you are pushing the stick/yolk away from you ie you'll be tipping the nose down. Conversely, if you pull the wheel TOWARDS you its as if you are pulling the stick/yolk towards you ie you'll be pulling the nose up.
Slow flight usually needs nose up trim (to increase the wing's angle of attack to maintain lift, not that I am thinking that in the air
). If you want to pile on the knots in the cruise you will need to apply nose down trim. Try practice in the car/bath - honest!
Other than that, remember to let the aircraft settle when trimming - it can take a little while for it to gain a natural equilibrium, if you are still trimming whilst its doing that then you won't trim for straight and level.