There has been a lot of debate over this topic. It usually boils down to preference. A neat ratio I have tried more than once is as follows:
2 parts tequila, 1 part fresh lime juice (yes, from a lime), 1 part lime juice (from concentrate, perhaps a lime-aid of sorts or a lime juice mix you like) and 1/2 part Cointreau or Grand Marnier (no Triple Sec unless you have no money and you are using Quervo ;--), and 1/4 part simple syrup.
Lately, I have been getting into making my own simple syrups for Margaritas. They fit in nicely in place of added sugar or orange flavored liquors or, in my example above, in conjunction with. Just dissolve 1:1 warm water and sugar, let cool, and bottle for later (be careful, though, long storage will lead to spoiling).
I like Bob's advice above, and adding OJ to margaritas is very good. One final trick I learned is to add a bit of pomegranate juice for color and tangy-ness.