This is an authentic Mexican drink. Don't be surprised by the lack of triple sec. Most Mexicans have never heard of it. I think American restaurants and bars added it to make the drink more sweet. No need for a fancy glass - just use a regular tubler or old-fashioned glass. I prefer a salted rim but they certainly aren't mandatory. To salt, rub the rim of the glass with a lime wedge and dip gently into some salt poured onto a small plate. It's best to do this well ahead of time to allow the salt/lime to harden. This keeps the salt on the rim and out of the cocktail. Tweak the lime and nectar levels to suit you taste. If you can't find agave nectar, fear not. Just use simple syrup (1 to 1 water and sugar boiled until dissolved and cooled).