Somebody asked me the other day how I come up with all these playlists - there's 300+ on this site! I thought about it for a minute, and the answer is that I just get super bored in the gym if I don't change out the songs I listen to often - like every few days.
My favorite source of inspiration for hip hop, reggae and drum n bass music is BBC 1xtra (I look through the playlists of the djs). For pop & rock music, it's probably iTunes (start with top 10 and then look through the playlists with a song I like); and for dance/ electronic music - Discobelle.net. The latest music website I bookmarked was Play This Song Loud - which is where I started when I was making this workout playlist - looking through the blog and coming across Painkiller KouncilHouse Remix (Pendulum vs Freestyles). Right now I have 8 unfinished playlists in iTunes, so when it's time to make another workout mix, I already have a starting point.
And that's a little bit of insight into the process I go through in case you ever wondered. And in case you didn't - just enjoy the workout playlists then :-)