I use 'jute' string, basicly it is a string sold for tying up tomatoes and has a test strength of 7lbs and is kinda coarse but not abrasive. I've also run into space problems as expected the plants to double not triple in height. I took to bending the branches over, kinda sounds scary when they make a large 'snap', but it wil just form a knuckle where you bent it over. For the next few days I push down on the limb as it natrually tries to reach for the light. A thing to remember is that you don't want to bend it too far down below the rest of the canopy as the plant gets confused on what is supposed to be the main cola and puts its flowering effort to the bud sites that are higher up.
In my cabinet I've used string to the walls and attached with a piece of duct tape, ofcourse I have painted walls. I've also run ties back down to my flowering container, a few holes drilled into just the nonessential lip of containers and planters provides an excellent place to tie to.
My current grow is touching the light in a few places and the link is in my sig........