The "Avoid Slips with Flaps Extended" placard is there to prevent you from stalling the horizontal stab, resulting in an aggressive pitch-down close to the ground which obviously you don't want. However I have never felt this happen even with full flaps and a full rudder slip. Go up and try it at a couple thousand feet. I bet if you got slow it would. Has anyone else had it happen? The previous post about airspeed in a forward slip is right. If you start at to high speed, the airplane won't want to come down.
Also remember that the slip can be used for emergency altitude loss during a forced landing. Use is anywhere in the landing pattern if you are really high. On base, lower the wing towards the runway and hold it with opposite rudder. To turn final, just swap rudders. Once the turn is complete, put the opposite rudder back in.
When you are ready to take the forward slip out, all you have to do is relax the rudder you are holding until the nose is straight down the runway. I teach people to count to three while they slowly neutralize the rudder. This makes a nice smooth transition that won't make your passengers puke, if they haven;t already.
