This recipe was given to me by an alcoholic pre-med student and produces some of the best frozen margaritas I've found.
In a 40 oz. blender put:
Blend until smooth.
Frozen margaritas may be stored in the freezer for later use. A quick dethaw (about 5 minutes on the countertop will do) may be necessary.
* The crappier the tequila the more sugar you add. If you use good tequila don't use sugar.
** Triple Sec, Cointreau, or Grand Marnier are fine examples.
This particular mixture might knock you on your ... fairly quickly and without notice because they are quite sweet.