A tripod is essential for static shots, but what about when you need to move around? It's difficult to get steady shots when you hold the iPhone from the sides.
 
The Joby Gorillapod Go-Go makes a nice hand grip and improves your tracking shots. Just squeeze the three legs together and you're good to go. Look out, Bertolucci!
 
Of course, if you want to spend more than $30, you can try the Zacuto Zgrip iPhone, which is more like $300:
 

ZGrip iPhone from Steve Weiss, Zacuto USA on Vimeo.