Completely replace the iron plates with bumpers for the majority of the weight on the bar. You can add smaller iron plates (5s, 10s, etc.) to fine-tune the weight on the bar, but the only stuff to touch the ground should be bumpers if you intend to drop the weight. You should have much more bumper than iron on the bar so that you don't lessen the lifetime of the bumper plates.
Also, you could just put the weight down: http://vimeo.com/25531302 Don't waste a lot of energy lowering it slowly, though. Like in this video, drop it to your thighs, and then just put it back on the ground.