Technical solution (damping pads, door closer) seems like a good one, though it might alienate your roommate if there isn't consensus on implementing it.
Often door-slamming isn't intentional, just a side-effect of the way the door closes (either it slams naturally or has to be slammed in order to close reliably and completely.)
The best way is to talk with your roommate about that and approaching it also in a friendly way like: 'I always forget to close the doors smoothly, what do you think about buying some damping pads?'.