To fill your background with realistic details, why not save some time by repeating elements? Whether you want to reinforce a sense of scale, or your image calls for repeated objects - "Smart Objects" can save you a lot of time.
This is a 4 part mini-series, and the pace is a bit different than other videos on this site. Instead of explaining a single topic, this mini-series serves to showcase fundamental skills working in harmony. So as you watch, you might see some techniques you're not familiar with -- but don't worry! The links to the right will catch you up to speed on these previously explained concepts.