How many times have you sat crocheting with a skein on each side of you, just daring them to fall or cross lines (paradox!) and going crazy because you don't even want to get up to take a break for fear of the consequences. Here's a simple solution I've been using for a few years now.
Materials: Lidded box, 1/2 inch rubber grommets, Dremel. (if you don't have one yourself, ask around, someone will!)
I got this cool divided box from Ikea for $9.99. But you can use any lidded box that will fit two yarn skeins in it. Make sure they don't fit too tightly, you don't want them jammed in there. You want the yarn to be able to come out of the skein freely. Take your yarn with you when you shop!
The 1/2 inch rubber grommets are from Home Depot, I'm sure they have them at most hardware stores. These were in the plumbing section, not that I pretend to know why.
First trace the inner hole of your grommet where you want the hole to go with a pen. Then use your Dremel to burrow out the whole. After a few trials, I found this easiest by starting in the middle of the circle and slowly moving outwards to the traced circle. You want to go just a little bit bigger than your traced hole to make getting the grommet in easier, but just a smidge. You can also do it the mathematician way and actually measure out a 1/2 inch hole and be all precise about it. Pinch the grommet, stuff it in the hole, and voila! A nice safe hole for your yarn to come through without being damaged or snagged.
I put two holes side by side on the long side of the box for shorter skeins, and two closer together vertically on the short side of the box for longer skeins. The ones on the ends can be used for the longer skeins stacked on top of each other rather than side by side like the shorter balls or skeins.
Now it'll be easy to set your project aside and even travel with it without those evil crossed lines and tangled skeins. Have fun!