Alix, I use this recipe that I got from Cook's Illustrated. I have never had a better tempura batter. It is soooo light and sooooo crisp! If you go to their site, you can probably read their whole story about how it works so well, but here's the basic recipe in my own words.
Shrimp Tempura
Clean & devein colossal shrimp (I used U-8's). Make one or 2 small slits in the inside edge of the shrimp to keep them from curling too much during cooking. Pat dry and set aside on paper towels.
Heat oil to 400F.
Mix dry ingredients in 1 small bowl:
1 1/2 cup flour
1/2 cup cornstarch
Whisk in another small bowl:
1 cup vodka
1 egg
When the oil gets to 385F, add 1 cup seltzer water to the wet bowl. Then whisk the wet into the dry. Whisk our most of the lumps, but don't worry about small ones.
By now, the oil should be at 400F.
Quickly dip the shrimp in the batter, then drop in the hot oil. Fry for 2-4 minutes, depending on the size, until just barely turning a pale gold.
Drain on paper towels.
Don't fry more than 1/2 - 3/4 pound in a batch.