To power slide in the campaign, you must press "crouch" while you are sprinting. For me, setting sprint to left shift and crouch to left control (which may be the default, I don't remember) makes this fairly easy. I just flatten my pinky finger and all is well.
That said, left shift isn't my desired location for sprint.
It has been suggested that sprint be made a toggle (like Borderlands or I think Call of Duty), which means you could start sprint with a push of a button then release that same button and continue sprinting as long as you are pushing forward. Let us hope this is implemented since it will make both our lives easier.
