There are two ways to go, IMCO.
You have your straight up margaritas, which are supposed to be VERY dry and you have your frozen margaritas that are on the sweeter side.
A) For the straight up margaritas:
1½ oz tequila (100% agave, white or "silver" and I like Cazadores best...the reverse sticker shock is also very, very pleasant.)
1½ oz lime juice, about one fat lime's worth, or two little ones. There's really no getting away from the fact that unless you are using freshly squeezed lime juice you're going to miss out. You'll also need the spent lime shells
1oz triple sec (I reallllllly suggest Patrón Citronge; again, more reverse sticker shock is nice)
Add all these, and the spent lime shells -- they'll give off a bit of their nice oils -- to a shakerful of crushed ice. Shake until the vessel frosts over. Strain and serve in a salt-rimmed margarita glass, or over ice in a salt-rimmed old-fashioned glass. Repeat as needed.
B) For the frozen margaritas:
1½ oz tequila (while 100% agave is best, in a frozen configuration you can get away with plain ol' Cuervo or Sauza, but don't get any cheapo store brands distilled in New Jersey)
1½ oz fresh lime juice, about one fat lime's worth, or two little ones.
1 oz triple sec (Notice this has more triple sec, in a frozen drink, you need the extra quantity to give you the flavor boost to compensate for the palate-dulling coldness. If you have it around, you could use Cointreau; I wouldn't go with Grand Marnier, I don't like the way its cognac base squares off with tequila.)
½ to 1 oz of sugar, depending how sweet you like it. Really frou-frou recipes will suggest you use agave nectar. Meh. You COULD make a 2:1 simple sugar syrup* if you wanted to speed things up, though.
Put all the liquids and sugar in the blender and get the blender going to dissolve the sugar. (This is key) Add about 6 oz crushed or shaved ice and blend JUST until it's, um, blended.
If your fridge doesn't dispense crushed ice, you can get a vintage ice crusher attachment for the Oster blender (my choice for all one's blending needs) on eBay for maaaaaaaaaaaybe $8 or you could use that Snoopy Sno-Cone maker gathering dust in the attic.
Serve in a margarita glass and repeat as needed.
Hope this helps.
-Joe, who spent his 4th of July party manning the margarita station
* 2 parts sugar, 1 part water, brought to a boil, then turned down to a simmer for 5 minutes, then allowed to cool off heat.