I guess I kinda cheat on this stuff, as I work in a DJ shop and get next month's radio singles this month, but if you're looking for more underground stuff that will emerge pretty soon, it might be a good idea to start following music blogs. Most of the music blogs will have links to other blogs, so starting with a few, you can easily build a network of a couple hundred that drop stuff you like. Some of my favorites are:
http://aurgasm.us/
http://www.cocaineblunts.com
http://www.sixeyes.blogspot.com/
http://www.tangmonkey.com/blogs/music/
http://www.fluxblog.org/
go nuts!