what i do is put the p plates that the rta give you just ehind your number plate and then put a cable tie through the screw holes in your number plate and on your p plate, that way its on the outside, can't come off, is clearly visible and people can't steal them
With the magnetic plates, I've been taking them off my car anytime I am parked - I just put the plates on the drivers seat, then I know that I won't drive away without having put them on again.