One good trick is to cable tie the clutch lever right in against the handle bar (as if you were pulling the lever in to change gears) then leave it cable tied overnight like that. This helped a lot to free up my clutch plates and definatly improved that sticky hard to pull in clutch thing. Oil change may also help some, try draining it as well when you cable tie the clutch in and leave it draining overnight too.
Once you remove the cable tie and replace oil, run the bike and then when you come back you could even cable tie it again (without draining oil) and leave overnight after all the fresh oil has gone through the plates to give it every chance of freeing up? (sort of like rinsing out the plates with fresh oil)