Trimming the airplane is something that comes with practice. Like many others have said here, the best way to learn is to get the airplane relatively stabilized where you want it (climb, cruise, or decent) and trim the forces off. The best way to get it right on is when you think you've got it close, simply let go of the yoke. If the nose comes up or goes down, feed in a little more trim in the right direction. You should eventually be able to trim the airplane to be rock steady in any flight condition (smooth air assumed:)). In fact, you know the trim wheel on the 172 has little bumps on it just for grip? You should get to the point where moving the trim wheel one bump in either direction makes a noticeable difference to the trim.

I tend to be very active with the trim. In fact, I usually retrim for nearly everything I'm doing if I'm going to be doing it for more than a couple of minutes. For example, on climbout, I trim for the climb airspeed and let the airplane fly the climb. In cruise, I do the same. Also on approach, I retrim for each configuration to give me the decent rate I'm looking for. In this way, I'm only ever maneuvering the airplane away from the trimmed condition. In other words, on approach, I don't have to "fly" the airplane for the approach because the trim is taking that workload. That allows me to concentrate on making the correct turns, setting up the approach, watching out for traffic, configuring the airplane, watching my decent rate, all without having to manhandle the yoke. If I need more or less decent rate, I just move the trim a hair.

The trim on the Piper Cub is a great little arrangement that uses a winding handle like a car window. It takes quite a
few turns to trim from a climb to level flight, etc. but it also allows you to make really fine adjustments. In fact, when in the glide for approach, I may make adjustments to the trim that are all of about 1/8th of a turn or even less.
Shawn

Trim Opinion
My instructor recommended the following and it works for me;-)
--First NEVER fly the plane with trim wheel, always establish your attitude (level, climb or descend) with Yoke and power
first.
--When you're stable trim off heavy yoke pressure first and resettle
--Then fine trim until you just have your hand on the yoke but you aren't inputting any effort at all.
--Let go for a second or two and check and check that it doesn't climb or descend is the final proof.
--Retrim for every change in attitude. I got to practice this in the circuit under the lashing tongue of my instructor... Trim
for climb out before and after raising the flaps, trim on crosswind after leveling, trim after every flap setting and throttle setting on downwind, base and final. You really get the hang of it then I can tell you!

Gene's addendum:
Should you ever be in a situation where the elevator is jammed and will not move, you should be aware that this causes the movement of the trim to give reverse effects in so far as directing the nose up or down.

Trim Opinion
Trimming takes some practice but once mastered will make every aspect of your flying easier. First thing is you can't trim the plane until it is where you want it to be. If it's climbing you can't just throw in nose down trim, that would be trying to fly the plane with trim and even though that's not that uncommon, it won't work. You have to hold the plane in whatever attitude you are trying to hold, let the plane settle up, determine whether you are pushing/pulling on the yoke and then trim those forces off. Of course if the forces are excessive trim most of it away, wait for the plane to settle and then finish trimming. The test is being able to let go of the plane at any time and the plane is still doing what it was doing before you let it go. Students never seem to see the importance of trimmed, hands-off flight until they have to either triangulate their position during a lost procedure or divert to an airport other than the one flight planned. Trying to fold and unfold those damn sectionals without being able to take your hands off the yoke can be a handful. On my private checkride the DE expected me to let go of the yoke, check for traffic, draw and compute to my diversion, check for traffic, maintain a shallow turn as I held altitude and check for traffic.
Stephen Ames