BrakClean is the best. Spray on a rag and wipe the rotor and pad. I have rim brakes and am not sure first hand of the benefits of scuffing the pads off the bike so someone else will have to speak on that. However, BrakClean has stopped squeal on my rim brakes without having to adjust toe when they get "contaminated."
On car disc brakes you just have to bed them with several stop from 20 MPH or so to prevent squeals. I am not sure if bike brake pads are the same or if your really need to scuff them with sandpaper. Someone else will need to comment on that who has first hand experience for bikes.