Take the clutch cable of, first of all. You want to get that out of the equation. If it still slips, you could have a bent clutch pushrod that's holding the pressure plate off (unlikely, but possible). You could have lost compression in the engine somewhere, and that would give the feeling of clutch slip. You could have oil that's too "slippery" in the primary case (have you used an additive?). You could have too much oil (suggest you drain the primary and try it for a while; it won't hurt to run it with oil in the primary for a day or two). You also want to check the clutch operating mechanism inside the gearbox end cover. Ultimately, you'll just have to drain the primary oil, take the primary cover off, and look at what's going on when you kick over the engine. The main thing is that none of these problems are particularly difficult to sort out. You just have to be methodical and work your way through them one by one. If the engine is running okay, you might try putting the bike on the centre stand, running the engine and using the back brake to see if the revs drop when you brake (MAKE SURE YOU KEEP THE REAR WHEEL OFF THE GROUND!!!). Better still, just ride the bike and snatch at the throttle. That will soon tell you what state the clutch is in. You could simply have worn clutch plates, of course.
