It may be cold outside but a good margarita is never out of season!
I've heard that you shouldn't waste good tequila on a margarita, but I respectfully disagree. Sure if you're using store-bought marg mix, full of high fructose corn syrup and preservatives, you'll be wasting tequila, but a proper marg recipe highlights the finer points of a good tequila.
There are many ways to make a good margarita, but usually the simpler the better. As Sterling Archer says, there are only five ingredients in a margarita: Tequila, Cointreau, lime juice, ice, and kosher salt. While I don't disagree, I like to put a little twist that complements the tequila and makes the drink a touch sweeter: Agave syrup.
1 1/2 oz tequila
1/2 oz triple sec (Cointreau if you have it)
1/2 oz Agave syrup
1/2 oz lime juice
Shake ingredients with ice and pour over fresh ice (optionally crushed), and garnish with a salt rim and lime wedge.
I usually omit the salt rim as the additional mess isn't worth it in my opinion, but your tastes may vary.
Interesting tidbit about agave (and tequila). The agave plant takes takes several years to mature and reach optimal yield. Blue agave, the only species of the agave plant that can be used to make tequila (liquor made from other strains of agave are mezcal), has been getting higher and higher demand as tequila rises in popularity. To meet this demand, agave farmers have been harvesting their plants earlier, resulting in less yield per plant, meaning more plants have to be harvested. This ever-increasing cycle has some worried about a potential crash in the agave market in the near future. Please enjoy your tequila (and agave syrup) in moderation!