What's the secret to getting restaurant-quality frozen drinks 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.