If you shred leaf that is the right 'case' (ie neither too wet nor too dry) then they are pretty much self-cleaning. But , hand on heart, if you are shredding a kilo then the chances are that some of the leaves will be a little too wet in places. Uniformly drying leaves is almost impossible for home users.
At some point you WILL end up with a shredder looking like this :
My personal favourite method is to reset the combs and then let it clean itself. How do you reset the combs? Simples. Turn the shredder over and using a small screwdriver press down on the comb tooth of the gunked up channel. If you can press it down onto a clean spot of the channel that is best. Once the comb is reset then you turn the handle and the comb will 'chisel' up the crap. A small steel/brass brush will clean the cutting teeth.
The Water MethodLet the shredder dry then remove the plastic cog housing on the side opposite the handle. Two small cross head screws. Then plunge the shredder into a bowl of HOT water and scrub using a small wire brush. Fully submerged keep turning the handle etc . You can add a little washing up liquid to the water but make sure you rinse well after.
Once you have scrubbed your shredder clean then roughly dry it- I use a cheap microfibre cloth and by shaking it. Then put it on top of a HOT radiator OVERNIGHT to dry (taking care that the plastic handle don't get too hot). Once it is bone dry then apply a smear of grease to the cogs and screw the cover back into place.