I'm partial to sangrias that have the extra kick of hard alcohol (e.g. whiskey, vodka, gin, brandy, rum). Lemon and orange slices are a must, of course, and you can add any other juicy in-season fruit you want. Keep the proportions as follows and you really can't go wrong:

For each bottle of red wine, add:
2 shots liquor (maybe tequila since it's Cinco de Mayo)
2 cups club soda, tonic, ginger ale, or any flavor of fruity soda (Hansen's works very well)
3 citrus fruits, sliced thinly (more orange than lemon or lime usually tastes better)
1-2 cups of other fruit (try kiwis)
2 tablespoons sugar (or less if the added carbonated drink is already sweet)

It's hard to go wrong, really.