I do continental knitting and hold the balls of yarn up close to me tucked in beside me on the chair so they don't go rolling around. I also, although not recommended by Alastair, keep the yarns tensioned. So I think this all keeps them pretty much in line. Then as you keep the yarn that isn't seen on either side in the middle of the three strands I simply watch to see how the yarn that needs to move needs to travel in order to not become tangled. Once you begin knitting it really isn't as hard as it sounds. And then when or if it does start to tangle because the balls of yarn haven't moved it is really a quick fix to uncross them and get them all back in line!