Dental floss would work, but I prefer twine. It's cheap (comes in a big roll for >$5) so if you cut pieces the wrong size, who gives a shit, just pitch it and try again. Just try and be nice to the nodes. You can use each piece a few times before it gets all frizzy and worthless on you. Twine is also not as fine as floss, which I would consider a good thing. I am by far no expert but am dabbling in LST and give plain old, Home Depot twine 2 thumbs up.