I agree with most of the fine guys here suggesting replacing a broken line, but what I generally do with a line that is broken in the middle due to friction with other pilots lines or some sharp obstacle (you know, trees, columns, things like that) is repair it using a blood knot (look for instructions here: http://www.netknots.com/html/blood_knot.html
), it is easy enough to be tied on the field and it could save the day if you don't happen to have a spare line at hand. Then I usually leave this line set in my bag as a spare or to donate it to some beginner pilot to get him out of those heavy dacron lines that come with some beginner kites. Anyway, in my experience, this knot is very resisting once you learn to tie it well, particularly if you have a lighter at hand and can melt the ends once tied. I've seen one of these lines I repaired being flown in quite strong winds without slipping or breaking the line again.