Most dealerships carry a special spray can, which you spuirt down into the manifold. The contents inside the can must be a heavy degreaser, but it foams up inside the manifold. I've never had it done but I guess it works. not sure I want to fill my manifold full of foam spray and then push all that crap through the pistons.

If you want to avoid the oil coking issues that are associated with PCV systems you might want to make your own oil trap.