Bottles of rat poison spotted behind the bar at Trump's Beach Bar in Atlantic City, NJ

Don't get sucked into buying a margarita "mix".  There is no such thing.  The only stuff that this nonsense will add to your drink is high fructose corn syrup, some chemicals to trick your mouth into thinking you're drinking something citrusy, and some other stuff that you certainly shouldn't want to put into your face. I should note that I don't think high fructose corn syrup is a "bad" thing in and of itself, but rather its existence in a food product is a good indication that the food product isn't very good.  I also think that those with strong opinions one way or the other are crackpots who have come to an uninformed, emotional conclusion, and then read things that support that conclusion.  

Limes and lemons are, last I checked, readily available.  And sugar?  We all have that as well.  That's all sour/margarita mix is:  sugar (well, simple syrup usually), and citrus juice.  That's it.  You can do it. Really you can.  And you only have to bother with it if you like your margaritas with a touch of sweetness.  I go both ways, depending on my mood.

So here's the deal.  This is it.  Nothing more:

• 3 ounces tequila
• 1 ounce Cointreau 
• juice of 1/2 lemon or 1 lime, to taste (update: this should actually read "1 ounce" as lemons/limes are very variable, to commenter Daniel's point below)
• a dash of simple syrup, or agave nectar, or neither
• a pinch of salt

• Add all ingredients to a shaker filled with ice
• Shake until very cold
• Strain into glass filled with fresh ice

A few notes on the above:

All proportions are a matter of preference.  The important thing is the actual ingredients (kinda like cooking, yes?).

The tequila is important.  Do not use Cuervo Gold, regardless of how much you think your local bartenders must know about drinking.  It's crap. And my gag reflex still kicks in when I smell it, due no doubt to experiences during my misspent youth.  Good tequila will alway say "100% agave" on it.  Right there on the label.  Sauza makes a plata (under their "Hornitos" label) that to my mind is a tremendous value, especially when buying the 1.75 liter bottles.  Start there.

The triple sec is important.  Do not use crappy, cheap triple sec.  Use Cointreau.  It's a real product, and not loaded with artificial colors and flavors.  As far as I know.  And it tastes better.  If you don't believe me, do a taste test.

Lime or lemon.  It doesn't matter.  If lemon is good enough for Maria's in Santa Fe, it's good enough for me.  But more importantly, lemons are generally cheaper, and their prices seem more stable, and both make a good drink.

If you don't shake at least 30 times, the drink won't be cold, and beneficial dilution will not take place. You do want to incorporate some cold water into the cocktail, via shaking.  If you don't shake long and hard, and especially if you put that ice from the shaker right into the glass with the cocktail, rather than straining over fresh ice, you'll end up with warm ice, melting very quickly in a warm drink.  Too much dilution + warm drink = not preferable.  

As for the salt in the drink?  There is/was a bartender at the Old House in Sante Fe, who convinced me to try a margarita his way:  a pinch of salt *in* the drink.  I was hesitant, but from that point on, I was hooked.  Give it a try.

A margarita at The Old House, Sante Fe, NM, with camera artfully cockeyed.  Note that this cocktail is not neon green.

And don't get me started on Rose's "Lime" Juice.  I've already been down that road.