If you find that holding the plane nose-up and adjusting it in that position is a pain, you can adjust it with the plane level. Start with the needle backed out a little (rich), then slowly move it in, a few clicks at a time (leaning the mixture). Give the engine time to "catch up", needle changes are not instant, the engine takes a second or two to react. As you move the needle in, you should hear the RPM increase as the mixture leans out. Once it stops increaseing, you are at peak. If you move the needle in more, you will hear the RPM start to drop off a little as the mixture is now too lean. Back the needle back out, and set it a little on the rich side of max RPM. Now, pick the airplane up and point the nose up. If you hear the RPM pick up just a little, you're perfect. If the RPM drops at all, you are too lean, back the needle out just a little. If there is no change, you are right on the boaderline, so you should probibly back it out just a little.

If you don't want to pick the plane up, you can also give the fuel line a pinch. If the engine picks up RPM, that's good. If it looses RPM, then you are too lean. I find the nose up test generally more reliable, but I use both, depending on the plane.

On trainers, I frequently just lean it out until I think it's near peak, then back it out a bit from there. I don't need max power and staying a little rich is easier on the engine anyway.

All adjustments here are done at full throttle. Be careful, take your time, and watch that prop.