I found this on the web and pasted it into my notes section a while back. This technique has worked for me.
1) Set scope to Max Power
2) Parallax set to Infinity (this is very important)
3) Point it at a solid, light background like a white wall or blue sky. You don't want anything except a solid, light color to look at.
4) Start from the diopter all the way into the ocular and rotate out.
Don't stare into the optic, open and close your eye as you adjust such that you have no more than about 1 second open, 4-5s closed.
In short order you can get the diopter very closely adjusted such that it will be crisp at all but the finest areas of the reticle.
Once you're there, small adjustments are made and you should focus your attention to the smallest details such as 1/10th mil hash marks, square corners in the reticle, etc.
Flick your eye open, then closed. Look at the reticle picture and see if it's crisp. Do not keep eyes open very long! Repeat as necessary until you have the diopter adjusted properly.
Lock down the ring on your diopter or wrap it with electrical tape and lock it down. Perhaps a sharpie with a witness mark between the occular and diopter ring. Something to reference it for the inevitability that it will get bumped.
