I'm not responsible in any way if you hurt yourself or ruin/damage your motorcycle. You've been warned!
Here's how to port (increase the airflow) of your cylinder head. You can do it yourself this way or spend $250+ for similar results. All you need is about $20 dollars for tools and the skill to take off your cylinder head or cylinder if you have a 2-stroke.
Buy a small file set (swiss cut preferably) "needle" files about 5 or 6 inches long. You should be able to get a set of 6 for about $7 dollars at your local hardware store. The cheap ones will work just fine.
Take off your cylinder head and clean the intake and exhaust ports with solvent (ports, not valves, although you can clean those if you would like).
Look inside the intake port as there will likely be some small deformities (they look like lumps, but not very high) on the sides on the port. They will probably run in a line, the textured surface of the intake port is normal. It allows the fuel and air to mix because of the slight turbulence around the walls of the port.
With adequate lighting, choose a file that will fit the area with the deformity you would like to smooth out. You just need to bring the casting deformity down to be level with the rest of the sides of the port. Slowly file the deformities away, until they are gone. DO NOT POLISH THE INTAKE PORT.
Exhaust porting. Clean out carbon build up with solvent and a fine scotch brite pad or a green one if you want to save time.
Choose a file, and remove the casting flaws (deformities) in the exhaust port.
When the flaws in the EXHAUST port have been removed, you may polish the port with sand paper. Make it as smooth as you would like. You can also match the exhaust port to the header and gain a few extra horsepower just from that.
Good luck and remember that you will be doing this at your own risk. I followed these steps and it helped my bike work a little better. It also worked well for a Yamaha dealer mechanic's KX250. This technique worked well for both 2 and 4-strokes.