Ingredients: tequila, triple sec, lime, sugar syrup, coarse salt.
Rub the rim of a margarita or cocktail glass with a wedge of lime and dip into salt; drop wedge into glass. Shake, with ice, one jigger of tequila, a third of a jigger each of triple sec and sugar syrup, and the juice of one small lime (or half of a big one). Pour, with ice, into cocktail glass. For something more refreshing, add a shot of water.
Blend the ingredients with enough ice to make eight ounces for a frozen margarita. Use four large strawberries instead of lime, half the triple sec, and blend with ice to make a strawberry margarita. Replace some of that strawberry with mango to make a strawberry-mango margarita. Use blue curacao instead of triple sec (and no strawberries!) to make a blue margarita. Lemon instead of lime is nice for a change. Experiment with other fruits or liqueurs--I have had many strange successes, including peach (where the triple sec is replaced with peach schnapps). Warning: Once you have made these drinks with real fruit you will never be able to drink those pre-made "mixers" again.
Many bartenders will tell you that real margaritas are made with sweetened, bottled lime juice. These are good too, if you replace the lime and sugar syrop above with one jigger of Rose's sweetened lime juice. Be sure to use Rose's lime juice: the cut-rate stuff is much worse, and all commercial margarita mixes are hideous.
My preference is to use very good tequila (I like Sauza Commemorativo) and make it unfrozen. For frozen margaritas: if your blender has trouble crushing ice, make more, not less; my blender will make four frozen margaritas at a time, but if I try to make only two I am left with chunks of ice. Perhaps the god of cocktails is trying to tell me something.