Err Am I mistaken or are some people confusing crabbing and sideslipping? Sideslipping involves slip whereas crabbing doesn't (at least not normally)...
Crabbing involves adjusting the heading to correct for crosswind (eg so the ground track equals runway heading). The yaw string should remain in the centre as will the controls once the heading is correct. It's relativly safe as the plane is "flying normally".
Side slipping involves making the plane fly sideways by the application of yaw. It usually requires some control input to maintain the slip or the plane "flys out of it". Some planes require "opposite" aileron to hold the leading wing down and prevent the plane rolling in response to the rudder input. The stalling speed is increased so there is some risk. Keep the speed up but not too high or the rudder/fin will fall off.
..at least that's how I understand it.
edit: Oh well I guess I was hasty posting this. Just read this thread again and it seems nobody is confused. I'll go back to my beer.