Take a couple spoons and slide the edge of the spoon bowls underneath the knob. Then slowly push down on the spoon handles causing the spoon bowl to rotate and lift up the knob til it pops off. This way only the smooth bowl of the spoon touches the guitar's surface and you won't accidentally leave any scratches or dings.
Use two spoons and a cloth. Put one under each side of the knob and gently pry upwards. By using spoons and cloth you help prevent damage to the finish.
you can also use a heavy guitar pick, thats what i use...the thick end that you hold, not the tip and work it around prying up on the knob. this works great since it's plastic on plastic, it wont scratch anything.
If you take a t-shirt and slide just a bit of it under the knob and then wrap it around to get a solid grip on it and then gently and slowly pull up it should come off.