[musicale]

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.)

[rarestoma]

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?'.