What's the secret to getting restaurant-quality frozen drinks at home? A while back I purchased one of the Ninja blenders so my GF and I could make frozen margaritas at home.

So far, we have been pretty displeased with the results.  Instead of getting a "smoother" frozen drink, we end up with more of a margarita-flavored snow cone.  The ice is still very crunchy and it's possible that towards the end of the drink you drink up all the liquid and just have bland granules of ice at the bottom of the glass.  I've even let the blender go for quite some time on high speeds in hopes to break down the ice more, but no real luck.

I do want to mention that we don't make our margaritas from scratch.  Still live at home and my parents are not big on alcohol, so instead of having all the ingredients on hand we buy the pre-mixed margarita mixes.  (Although, we would prefer making them from scratch.)

So, what can we do to get better quality frozen drinks at home?  Are we not getting good results because we use the mixes?  Is it the blender maybe?  Or possibly the ice (we use ice from the ice machine from a fridge, so it's in pretty big cubes starting out)?  We go out and get frozen drinks all the time and love them, but have not yet been able to get anything close to them at home.
You might have better luck if you use ice that is composed of margarita ingredients (lime juice and water, I guess). Unless you're referring to margaritas on the rocks, many restaurants actually have something akin to a slushy machine that is used for margaritas, so there is no ice added to the drinks. Instead, the actual margarita ingredients are frozen while stirring to prevent adding anything that will water it down.