Chances are, this means you're using an overly sensitive or poorly calibrated joystick or your sensitivity settings are all wrong. Try adjusting the Dead Zone of the axes that are giving you trouble (move the blue tab in the Axis Edit dialog), more away from the center if it thinks you're pressing a button when you aren't, more toward the center if it thinks you aren't pressing a button when you are. If that doesn't work, try manually adjusting your joystick (if it has adjustment knobs/sliders), or try calibrating it with jscal.