I feel slowplaying is one of those things that is very dependant on your opponent as well as the board texture. If there is a flush draw or a straight draw that hits your opponents range I feel that the best play is to ALLWAYS protect your hand by pricing them out of the pot. You can't always get the donk to fold (who doesnt like to chase eh?) but you are still building a pot that you are most likely a 70%+ favourite to win on the flop.
Your opponents tendencies are very important as well, so here is where being observant comes in handy (a HUD is nice as well ofc). On a dry board you might not be able to get action by betting, and the slow play should then be used to try and induce value from your opponent.
In the end poker is about getting the best value out of your hands, whilst minimising your risk.
Oh, and I've not been playing that long so please, PLEASE correct or comment on this if you feel you have something to add, interject or just plain flush down the toilet