Ed,
I am no expert seeing as how I have never caught a single fish on a senko. One good resource though is at Yamamoto at:
http://www.insideline.net/ezine/ezine-1-1-senko.html
Lots of good stuff. If I have any advice it's "do nothing". As in cast out your senko and let it sink all the way to the bottom. Then leave it there till you are bored and move it a little. Seems to be the general strategy.

-Rob