Have you heard of the "tillering gizmo"? I recommend taking a look at the post below. This tool helps me greatly in tillering!
http://tradgang.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=125;t=001047Basically, it is a 1inX1in square block of wood, 6in long with tapered ends. You drill a hole through the center of the wood, and then a little bit bigger of a hole halfway into it that is big enough to bang in a nut. A wooden pencil will get screwed into this nut. You then put your bow on your tillering tree and start pulling it down to your draw length. Lay this block of wood flat on your bow and run it up and down the limbs, keep your eye close to it and find the spot that has the biggest gap between your bow and this block. Adjust the pencil so that it is just off of touching your bow. After the pencil is adjusted, run this wood up and down the whole bow- the pencil will mark your stiff spots and show you were you need to scrape.
Once you can get it to run a full pencil line on your bow without any breaks, your tiller should be close to perfect. You may still need to eye it though for some finishing touches. As well as keeping an eye on your fades if you have any.