I'd start buy checking the slack in your clutch cable at the controls. If fiddling with that doesn't work, take off the lowers and play with the cable at the clutch cover. If you can't get it downshift smoothly by adjusting the clutch cable, then it could be your clutch itself. I've got 23K on my 02 and ithe plates show almost no wear, but if you've been hard on it, haven't changed the oil frequently or have done lots of wheelies, it could be shot at 16K.