Hi, apart from selector fork damage(bike falling over on gear lever side)in which case remedial work will have to be done as per Adam's advice, but before you have to spend cash the only other fault which could cause poor gear changing is a poorly adjusted clutch and/or warped clutch plates, could be the cable is slacking off therefore the clutch is now starting to 'drag', same symptom for warped plates. Take out all but a small amount of free-play by adjusting the knurled adjuster at the handlebar lever (10-15mm free play at the lever's end) Maybe give the throttle a 'blip' when changing gear if you say it helps. Double check that the gear lever is tight on the shaft. All the best.
Chris P.Duck, December 2010