A friend taught me this but one of my favorite tools is a sock. Not a sock I knit, just a normal, ordinary, comes from the store sock. I put my ball of yarn in the sock while knitting and it prevents my cats from gaining access to it and attempting to rip it to shreds. (They have a particular love of Noro Big Kureyon...) It also makes good secure storage for the knitting project if it is small. (Although, I suppose it would look kind of silly carrying a bulging sock full of yarn around town...)
Ummmm. I could add a picture here but y'all know what a sock looks like. Heh!