paintingbynumbers has a full tutorial on creating mash-ups, remixes
that combine two songs. This is a 'beginners' guide' but it's quite
intense: author Philip Meehan covers everything from isolating beats to
removing vocals via phase cancellation. It's inspiring enough to make
me want to mash some of my own tracks.

Best of all, the entire tutorial takes advantage of Mackie's Traktion audio editor, which for a limited time is free, as we reported here last month. [Thanks,]