Without having the special tool, what is the best way to adjust the rear sight on my model 21? I finally had a chance to really shoot the thing the other day and noticed all my shots were striking left of the aiming point, grouped together but still left of target.

For some bizarre reason new glocks do that... many of the new guns we issued a few years ago all needed cranks to the right.
Anyway if you do 't have the tool here's what you cando...

First you did not state what kind if sights (metal or plastic) you had on the gun. You will need either a BRASS or PLASTIC punch/drift tool as well as a small hammer. Simply secure the gun in fashion in which it will not move (if you use a vise wrap the gun in heavy material to protect it) and tap the rear sight in the direction you want to the group to to. If you want it to shoot right, move the rear sight to the right.

Don't be too forcefully and take your time. The sight will be hard to move at first but you will just have to figure out how hard to hit the drift to get it to move. Make a small adjustment first and then go shoot it. Handgun rear sights do not take much adjustment for windage.

hope this helps..

A wooden dowel works to.

I recommend getting some aftermarket adjustble sights. I like the dawsons, they are the best drop in adj sights for the Glock. dawsonprecision.com

Have you considered trading the Glock in on a SIG or H&K?  That's always a really good way to adjust Glock sights!
Just kidding ya'!