Syn:
A big +1 to Chukitova. He has explained all the possibilities perfectly, and he is correct in his diagnoses. To inspect your clutch, lay the bike down on the opposite side to where your clutch cover is. This will allow you to remove the clutch cover (I believe it is the cover on the right side of the YZ250F), without having to drain the transmission oil.
Once the cover is removed, you will be looking at the clutch assembly. As Chukitova has said, you will need to inspect 5 things primarily. They are: Springs (4 or 5 of them, held on with bolts), Fiber plates (dark brown in colour), Steel plates (bright and shiny), the Inner Hub and the Outer Hub. Chances are, it will be the outer hub which is worn. This is the large round aluminum part with slots in it that holds the clutch plates in place. Over time the plates will wear the slots on the hub down. You may notice this when you look at it. I have seen some people file these so they are square again but filing enlarges the size of the slots, which makes for a sloppy clutch, so it is better to replace the hub.
As for the plates, you will need to check a service manual and find the minimum thickness for the fiber plates. Generally these are designed to wear out before the steel plates. You will find about 8 or nine fiber plates (alternating with the steel plates within the hub), and you will need to measure these with vernier calipers to check their thickness. If they are less than the recommended thickness then they are worn out and should be replaced.
The same goes for your springs. Once removed, measure their length, if it is less than they should be, they will need replacement. If they are borderline you can install a washer or two when you put them back in but chances are, they are probably tired and should be replaced.
Though it looks complicated when you first open up the cover, the whole procedure is quite simple to perform. Removing the bolts that hold on the springs will allow you to remove both the fiber and steel plates which you can then measure. This will also let you see the inner and outer hub surfaces to see if they are worn.
If you get the service manual there should be an explanation of how to do this. Only other thing is you may have to replace the gasket on the clutch cover when replacing the cover, as they can tear when separating the clutch cover from the crankcase.
So, depending on the age of your bike, you may be looking at replacing the outer hub, at least the fiber plates, and it would probably be a good idea to replace the springs regardless, as they are relatively inexpensive and play an important role in preventing the clutch from slipping. Once the job is done you will need to re-adjust your clutch cable with the correct amount of free play and life will be good once again, in fact, you may be able to roost your buddys, even in fifth gear !
