I usually get my sacks of crawfish from LACRAWFISH.COM and they include their powdered boil of which BOTH 1lb sacks go in the water. If you get them somewhere else, buy and use this stuff:

[Link: amazon.com]

Use at least half the jar if you get a sack of crawfish (30-35lbs). You cant really go wrong using the whole jar either. This won't make them super spicy. You need to add cayenne as well. And A LOT of it.