In a shaker, add equal parts lime juice and cointreau
Add a bit of agave syrup **(to taste)**
Add 2-2.5 parts tequila
Add ice and shake until cold
Serve with salt rim if desired
Since I'm squeezing fresh lime juice, I tend to base the rest of the ingredient proportions on how much juice I get from the lime
The agave syrup adds some sweetness and body to the cocktail, and pairs well with the other ingredients. I tend to use a quarter to a half as much syrup as lime juice.
[Espolon](https://espolontequila.com/) is a widely available tequila which I used for a long time, and while you could do worse, it's also relatively easy to do better. I particularly like [Fortaleza](https://tequilafortaleza.com/), which is a bit pricier, but much more flavorful.
It's easy to create variations on the standard margarita by substituting alternative liqueurs for the triple sec. I enjoy the "italian margarita", which subs amaretto.
To add more flavor complexity and depth to your margarita, try putting a bit of citrus peel into the cocktail shaker before shaking the ingredients. This technique is called the "regal shake", and works particularly well for margaritas with a grapefruit peel.