Okay, it's been about a year since I last tried restarting one of these. It'd be great if this took off. When this was last properly up and running I found a few songs that I wouldn't have found otherwise, some of them I still listen to now.
One poster provides the next with up to three songs, linked from youtube, soundcloud or similar. (To help prevent geo-locking, either wrap the name of the song in spoilers, or post alternative links.) The next poster then listens to and rates the song(s) of the above poster, either grading them (A+,A-, etc.), or giving them (each) a score out of 10. That poster then provides links for their own song(s), and it continues!
Preferably, try to post songs that not everyone will have heard. Obscure tracks are great. Try to avoid 'big hit' songs that's been a huge commercial success all over the world that everyone will know. So, although "Hotel California" by The Eagles is a great song, you've probably been living under a rock if you haven't heard it. Having said that, it's still good to post less well know music of well know bands or music from bands that have done well in one country or a few countries but not a significant amount of others. For my first three I've got one band that's done really well in the UK but nowhere else, one that's just been pretty successful in their home country, Australia and one (Finnish) that's done well across eastern Europe. That's not to say you can't post the odd huge song, just don't do it too often.