Best test to see if you've trimmed properly: Let go of the stick/yoke. Not just for a few seconds, but for five minutes or so. Watch the altimeter for any trends and trim in tiny, tiny increments, then watch the trend again. At the same time, small bank excursions can be corrected with the rudder. But try to keep your hands off the stick/yoke the whole time.
It will not work in turbulence and it's also a bit too coarse a method to apply in controlled airspace, but it's a very good training exercise. If you have height excursions several 100 feet up or down it's not just turbulence or not paying attention, it's also overcorrecting and improper trimming.
The other thing that you might be doing wrong is leveling off after the climb. Interesting piece about this in last months Flyer magazine. After leveling off, maintain full power and let the aircraft accelerate to cruise speed (assuming cruise speed is below redline RPM). Only then select cruise RPM, and trim. If you set cruise RPM immediately after leveling off, but while the aircraft is still accelerating, you'll be hunting the correct power setting and trim for a long time.