We use the technique Grogmonster outlined on the Twin Comanche. While for many aircraft it's natural to associate the rotation speed with the speed at which the decision to fly or land ahead is made, it just doesn't work practically for others. In the case of the Twin Comanche, it wants to fly a good 10 to 15 knots below blue line, and trying to keep it on the runway, particularly in a crosswind, is exciting. So we accelerate after rotation before climbing. I don't think it has to be "in ground effect" as such, as long as the action in the event of an engine failure can be to close both throttles and land ahead.