Getting used to running on trails rather than concrete/road running?

Also adding to your other awnser, shorten your stride while running on trails. Running the hills might be the biggest change, you said you mostly ran flat roads. The hills and trails will strengthen your legs and are speed work in disguise. Slow your pace till you get accustomed to the change. If you're planning on a lot of trail running a good pair of trail ruining shoes would be a good idea also. Have fun and RUN MORE!