The trick is to put a little water in the bottle first, before adding powder, then top up and shake. If I put the powder in first, then add water, about half the powder becomes one giant mass stuck to the bottom of the bottle and no amount of shaking will get it to dissolve. If I put a little water in first, no such problem.

Edit: for reference, I'm mixing three scoops (190 cal IIRC) into 24 oz. bottles.