A lot depends on when it happens. If it's the first shift of the day, it is a normal trait of wet clutches and easily stopped. Just put the transmission in gear, pull in the clutch lever and rock the bike back and forth untill the clutch breaks loose.
If it is most of the time, but only when you downshift, try goosing the throttle a bit as you shift to synchronise the gears.
If only on upshift, try shifting just a little slower or quicker, one or the other may help, and make sure to back the throttle completely off.
Other things that might help, warm the engine up enough to be able to push the choke in at least half way. If you don't have time, keep it adjusted so that the idle speed is as low as you can manage without stalling.
Make sure the idle speed, chain tension and clutch adjustments are correct.
If it only happens on certain shifts, it is a long shot, but you could have a bent or badly adjusted shift fork. What happens here is that one gear doesn't quite disengage and it is then violently kicked out when the next one engages. This can be a bad thing, a friend from long ago had the gearbox lock up on his (then) new Brit Indian when he shifted to 3'rd gear at about 60 MPH.
