I noticed here and there 3rd and 4th gear can be a tad bit difficult to shift into. When I say difficult I mean that sometimes there's a little bit of resistance, almost as if it gives a slight "sticky" feeling. Most of the time I don't even notice it, and when I do it's nothing very significant. Just something I picked up on. I've had my bike for two months and I'm not sure when the previous owner changed the tranny fluid, maybe it's time I should? I just don't want the transmission locking up on my one day..
The tranny fluid is the engine oil. If you haven't changed the oil yet do it right away!
Use a good MOTORCYCLE synthetic oil (Amsoil MCF, Motul 7100, etc.) and good oil filter (Amsoil EAOM103).
How long are you holding the clutch in when you are shifting? If you hold it in too long it'll be stiff or clunky going into the next gear. You should be kicking it into the next gear right as you're pulling the clutch in, split second shifts.