I've been swapping hard drives in my Mac Pro a lot lately, and they kept getting more and more difficult to pull out once they'd been soundly seated, to the point where I was in danger of bending the lip of the sled.

I was about to experiment with lubricating the things with a little graphite or similar, when I discovered that they slide out much more easily if you reach both hands inside the Mac and pull gently and evenly on the far edge of the drive itself.