Professional chefs cool the water, eggs and flour in a refrigerator before mixing them, to prevent the batter from becoming sticky. Deep-fry in lots of oil, and make sure the pieces don't stick together. Shrimp makes good tempura . White fish like sea bass and flounder do too. Squid is another good choice.
Some people use thin slices of abalone, although it is expensive.
Carrots, squash and sweet potato are three of many vegetables that taste great as tempura . Fresh shiitake mushrooms are an excellent idea too. Hot tempura tastes wonderful with a sprinkling of salt and the juice of a squeezed citrus fruit like sudachi or lime. Usually, though, the Japanese like to dip each piece in a thin sauce called tentsuyu. As soon as the mixture boils, set it aside to cool.