Grab an instructor for the side-slipping training - it's often left out of the PPL training (should be in descending part 2).
I'm not a big fan of slipping the PA28's as I don't think you get high enough rates of descent (even with full flap). Bigger rudder needed? I prefer S turns, particularly in the Cherokee which can sink really well.
I'm clueless about the aerodynamic forces involved - there are certain control surface blanking issues with side-slips in some aircraft (eg I believe Cessna's with full flap) but that would be written in the POH.
Main thing to watch out for is pitot/static instruments as airflow into vents is disturbed. Some ASI's will give false readings in the side-slip.
If poss. get yourself into a Cub, Tigermoth or even an Extra if you've got loads of dough. These slip really well.