A forward Slip is preformed to loose as much altitude without gaining airspeed.... A side Slip is preformed to keep the longitudinal axis of the aircraft aligned Parallel to the runway while in a crosswind landing....

They're both preformed the Same way.... The side slip you're just using the rudder to prevent the aircraft from turning in the direction that the wing is down in. The Forward Slip you're using the rudder to put the fuselage into the relative wind to produce maximum amount of Drag, Hence the lack of gain in airspeed