To get the best results, I recommend hitting Streamline with a stick.
Streamline's a good program, but I find it terribly random. If you're doing anything other than a character sketch or illustration, it may not be worth it (I have yet to get it to convert a full size comic page to a standard which pleases me). All I can suggest is tweaking the settings until you get one which works with the piece you're doing at the time. Although this is probably just me, and there will, I'm sure, be people out there who have used it a hell of a lot more than me and can offer some real help. Sorry. :P