Monster makers.com sells a product called water trap. I've used it and it works perfectly with alginate molds. Mix equal parts A B and water trap powder in to the resin and pour into the mold. Really cool stuff.
It's very easy with a few precautions, first, swill the alginate out with acetone. Doing thing with wick away the surface moisture, then spray a light coat of laquer over the alginate and when that's dry, dust some talc onto the surface, then put your resin in. The quicker th resin kick, the less chance of it inhibiting so you could always make sure the mould or resin was warm to accelerate the cure