Secure website login tutorials?

Hi,  I'm trying to write a bot to login to the scottrade trading site, 
and while I'm taking care to grok hidden fields
and I've initalized a cookie jar for my LWP agent, posting the login 
info to the form still just returns the login form
again (no error message).

Are there any tutorials out there that address common caveats for this 
sort of thing?  Or what else do I potentially
need to take care of to make sure my LWP agent is getting all of the 
necessary session data to log in?

Thanks,
-John