If you need to attach a camera to your kite or balloon line, you can use commercially available line clips, or you can just use a carabiner. Carabiners are easier to find and they don't weaken the strength of the line. Simply wrap the line a few times around one side of the carabiner and as long as the line is under tension, the carabiner doesn't move.
I've used this method successfully for a while on my lifting parafoil that flies at a fairly low angle, and recently we found it to be very successful for attaching a camera to a delta kite as well, which flies at a very steep angle (almost directly over head).
I may have originally seen this idea at http://www.gombergkites.com/line.html#car or possibly another site.