I no longer work with the DD population, but I did for 12 years. The best advice I can give is to expect more from them than anyone thinks they can possibly give. If you have low expectations, that is exactly what you will get. If your expectations are high, you might not meet them all...but I can guarantee you will see results.
Also, look at each person as an individual. Don't ever see a person as one of a group (Down's, autistic, etc). You must see each person as unique and individual. Whether their abilities are minimal, or they are highly functional, everyone has a key. It is your job to find the key to unlocking that person's door. In this way you will find what motivates that individual to achieve.
I hope this helps...Good Luck...You have chosen a challenging field. Enjoy what you do...but don't let it overwhelm you. The DD population as a whole are some of the happiest people I have ever met. We should all be so content with our lives...