... front ones are quite easy.. you can actually slide them back, then lift it up. The clips will remain in the door frame, and you can slide the sill back on afterwards.
The rear ones are a bit more difficult if you have a sedan. You'll need to punch the clip out of the retainer at the back, then use a flathead, or claw, to remove the pressure clip. Then you have to be very gentle to pull up and push forward at the same time... I still haven't mastered it. Replacement clips are available at the dealer inexpensively though. (I think it's like 40 cents each?)