The problem you discribe has three common causes.
1. The clutch basket is "notched".
2. The cable is improperly adjusted.
3. The plates are warped.
The easiest check is the cable. You should be able to put a dime between the lever and the perch without putting any pressure on the cable. Make adjustments at the perch or the sleeve to acheive the proper free play.
If you have checked the basket AND filed off the "notches" you can only expect a modest improvement AND you can only do this once. I have found that when you do this, you should "flip" the fiber plates over and insure that the tangs have no sharp edges on them. If the inner hub is notched, you will have to change it out. Filing the splines on the hub can be done but you will weaken the hub and will likely have a catastrophic failure.
A dial caliper and a perfectly flat surface is the only way to determine if your metal plates are warped. Excessive slipping will cause the plates to overheat (burn) and should be readily visable. The next thing to look at is the pressure plate. Determining if this part is warped will likly take someone with special equipment. This is probably the least likely problem and if it comes down to this, replace the hub rather than pay someone to tell you it is warped.
The next possible problem for a dragging clutch is the thickness of the clutch pack. Using dial calipers, check each fiber and metal disc in several locations. Write the measurement down and add them up. Assertain the maximum clutch pack thicness and see if you are within specs.
If you were experiencing slipping, I would check the springs but a dragging issue is a different problem. Finally, the actuator lever and push rod could be worn. This problem will not show itself until all of the other possibilities are elimenated. If this is the problem, a properly adjusted cable, plates within spec, and a hub/basket/pressure plate in good condition will show that the clutch pack is not spreading sufficiently to allow slippage with the cutch engaged.
Most of the things I've discribed can be done easily but an experienced mechanic maybe your best bet. Dirt bikes are fun but they take money to keep them in tip-top shape.
Good luck. If any of my discriptions are wrong, I'm sure other members will correct them. I don't claim to be an expert, just sharing my experience and this info is general and NOT bike specific.