I have owned the HD600s for some months now and have managed to successfully clean the pads twice.
What I do is quite simple:
1. Pop off the two pads.
2. Take a Q-tip, moisten it, and just scrape it around the bottom leather part to get off most of the gunk (this is where it builds up on my pads). Then, I take another Q-tip, moisten it, and scrape it around the inner workings of the headphone shell taking extreme care not to come close to the driver.
3. Pop on the pads.
Of course, if you have lots of gunk building up on the pad surfaces themselves, your best bet is probably to either lightly rinse them or just get new pads and save yourself all that trouble.
Cheers,
Geek