2 ounces blanco tequila (we've been a fan of Altos lately)
Juice of one lime, plus extra lime to garnish
5 ounces or so of grapefruit soda (Izze is great for this)
Cayenne pepper (optional but recommended)
Rim your glass of choice with lime juice, and then with salt or a mix of cayenne and salt. Fill glass with ice. Add tequila, lime juice, and then top off with grapefruit soda.
Now if someone could bring me chips and guac we'd be in business.