For Kinect, I use the same Repeater Cables (a.k.a., Active USB cables) that I use for my PS3 Eye cameras. These are different from standard USB extension cables in that they have an electronic component on then end that boosts the cable's power and signal. They're fairly 'cheap'--I get mine for about $11 or so at stores like NewEgg.
GWC 16.4 ft. USB 2.0 Repeater CableIn theory, you can daisy-chain up to five of these cables guys, which would be ridiculously long. I think the longest chain I've used for a single camera is three, but normally for PS3 Eye cameras I may have two cables per cameras, and for Dual Kinect Sensors I use a total of three repeater cables, one for the nearer sensor and two for the farther one.
For Kinect, because each device has its own power brick, the extra boost from these cables is for maintaining a steady data stream, not for powering the sensors. (Well, that's been my assumption anyway. If a more knowledgeable user knows otherwise, please correct this post.)
G.