They want to make sure that you intend to return to the UK at the end of your course.
Show them you have a home (mortgage?).
Show them you have a job. (Letter from employersaying that you are still employed on your return).
Return airline ticket.
Bank account still open.
Any appointments you may have m,ade for after your return to UK.
Anything to show your intend to return to UK after the course - and not just to stay in the USA as an "unlawful alien".