There used to be some threads on taking bungee cords to keep the doors open as they are heavy. If you use trash cans, you're always having to step around them and open the doors.
As the other posters have said, just let your room steward know and they'll bring you rubber door stops. I had read some posts in the past that they sometimes run out, so we always brought ours just in case. They're less than a $1 at Wal-mart or Target and very easy to pack just in case your steward doesn't have any extras.