Cutting line...downhauls, harness lines, whatever...can of course be done with razor blades, finishing the ends with a match or a cigarette lighter, thought the results are often less than great. As someone who likes tools, and who religously replaces and tweaks lines, I went in search of the perfect line cutting tool a few years ago. I think I have it worked out.
Murrays Marine sells a purpose-built tool that looks a lot like a soldering gun to me. If you already own a soldering gun you can just buy the replacement tip. I bought the replacement tip, and then went out to get Radio Shack's "Dual heat soldering gun with light" for about $25, and have been in rope-trimming bliss ever since. You laugh, but when I pull this baby out, all my friends come over and need "a quick favor." I'm packing heat.