a short in the starter circut i would bet. or a short in the started itself.

and just keep putting bigger fuse's in, until they won't blow.  but then again that's not so good on the wiring.  

Your starter takes the biggest power draw of anything on your bike.  If you're grounding out that circuit to your frame, you'll blow about any fuse you choose to put in - it would be worse for you if you didn't.  Electrical problems can be tricky - (had a lot of them on my old '67 Porsche 912 or ask any 60s V-Dub owner)  Be methodical and deliberate in your trouble shooting.  If you've got voltage passing from any isolated metal parts to a valid ground, thats the trouble location.  There are a lot of good automotive electrical trouble shooting sites on the web - go google crazy... As suggested, get a good multi-meter and start with the starter switch...good luck!