Bumped by your thumb? or by other external factors? I'd be worried if it's by other external factors as a bump significant enough to change your exp comp dial on a regular basis may cause damage to your camera in the long run.
I never really had any troubles with the exp comp dial. My thumb usually rests at the left edge of the hump where the Q and AE-L button is located when I hold the camera with the handgrip accessory. Perhaps you can try using a thumbs up accessory, to prevent accidentally bumping the exp comp dial.