I cook them the Cajun way. Season a large pot of boiling water (enough water so that when you dump the other igredients in, it will cover them and continue boiling) The best seasoning is heavy saly and cayenne pepper.
Once the seasoned water is boiling dump the crawfish into it live along with corn on the cob, cut up potaoes, and sliced onions. About a pound of live crawfish per person and at least 2 ears of corn each. After about 8-15 minutes of cooking everything should be done; remove from the boiling water and serve.
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The traditional way to serve it is to pour it all onto a table that has been covered with newspaper or masking paper. To eat them you pull the tail off and peel it like a shrimp (only the shell is tougher than a shrimp's shell) Once you've eaten the tail meat, you suck all the meat and fat from the upper end through the cavity left from pulling off the tail.
If you'r not into the Cajun thing, they're still cooked and served the same way; only you just eat the tail and skip sucking the head.