Open your concave as much as you can and slow the rotor down a little. Good luck though, we have never had a lot of success getting decent straw out of an STS. A crop with heavy straw and a little moisture helps, but normally we have neither.
Using the right baler is just as important as using the right combine. The Glenvar Bale Direct System has been successful in Australia and some units are running here in the US and gaining popularity. Collecting all the chaff and loose grain helps fill voids and creates a denser, firmer bale. www.glenvarbaledirect.com.au for some details and video
Most of these are behind Cat / Claas Lexion combines, some CNH shown in video, but with STS their plans to chop and blow Bio-mass may be the only option, given the MOG condition out the back.
We had over 400 acres of straw baled behind our STS last year, had no problems. I specifically asked the guy that baled it if he had any trouble and he said no, it worked just fine. He was running a Hesston 3x4.