If I sleep on my right side I get long, LONG dreams that merge with others but are very realistic, if I sleep on my left I get short, fleeting, nonsensical dreams.
Aside from that, I can't control the dream while I'm inside, like I can't just say 'Hey, that rabbit should turn into a rhino.' and then it happens based on my own concious will, but I do have the ability to control myself to the normal degree I can in reality.