To keep your line from getting into a ball, always make sure that either your line is fairly straight (with no circles), and If you are holding the kite, have one hand holding the end of the line, and the other hand holding the center of the line, and the kite. This will do a good job preventing knots.
I generally practice indoors at my mom's school (she is a vice principle), or at my own school in the lunch room. (I'm 15 years old, so I'm still in high school.