Yes, I will so long as its a smaller road, and within a mile or so of the trap. Otherwise by the time they get to it, they'll have sped back up.
I do notice people warning me, but you're right, its not as often. I almost blew past three cops today parked along a curve in ridge road (blew for a newbie... maybe 10-15 over the posted 35) and had just passed a car that didn't do anything...
As far as right way to go, I'd guess flashing the lights is universal? Maybe pat your helmet while flashing your lights and rubbing your belly? (And if you can do that, and ride a bike, I've got $5 for ya!)
And I believe there are laws that prohibit warning about speed traps, but am about to pass out so am not doing the necessary googling, sorry.
Life's like a machine gun... just close your eyes, squeeze the trigger, and pray you hit what you wanted.