What is an efficient way to play .rm files?

Install Real Alternative. This is a free Media Player Classic and Codec that will allow you to play RealMedia files (.rm .ram .rmvb .rpx .smi .smil) in Media Player Classic. It works great, and you'll never have to see that wretched RealPlayer again.Media Player Classic and the appropriate codecs are the solutions to ALL of your computer audio and video needs.

I would recommend VLC. The original Realmedia-Player always was crap (back in the days when I used it). Although VLC is relatively slim and simple, it plays a lot of different file types.

I have to add that .rm is only considered partially supported, but I don't remember having any problems.