Use liquid seasoning in the boil water - and use 2x whatever the recommended amount for your volume of water is on the manufacturer's package.
Get the boil water to a good roling boil, then dump your crawfish in. Boil until about 1/2 of the crawfish are floating (5-10 mins generally). Remove the crawfish from your boil and put in an ice chest immediately to allow them to steam. This is when you add any powdered seasoning if you want. I generally do, because the steam liquefies some of the powdered seasoning and because you get that good seasoning on your fingers while you are peeling the crawfish. Some folks also add a dollip of butter here. I'm iffy on it personally - somtimes yes, sometimes no.
Let steam in the ice chest for about 10 minutes or so, then serve.
I always put the sausage, mushrooms, new potatoes, onions and corn in with each batch. It's good to pre-cook them a little bit, especially if you have a good burner and can boil the crawfish quick enough that you don't get a good cook on your veggies and add ons.
A lot of folks will add garlic bulbs to the boil water - if you do this, make sure your seasonings (liquid and powder) are light on the garlic content. Otherwise I've found that you end up with too much garlic and it overpowers everything else (this coming from somebody that likes garlic)
No need to purge. Some folks still do it, but it's a holdover from the old days and not really required anymore. Most folks I know over in south LA and across cajun country don't bother with it anymore. It won't hurt anything though. Just soak in some good tap water for about 10 minutes prior to tossing in the boil water.
[This message has been edited by schmellba99 (edited 3/29/2012 2:22p).]