LifeButBetter.com brings us a simple fix to prevent tangled headphones:

To stop earphones from tangling we simply need to tie the ear-buds and the jack together. We can do this by folding the wire in half to bring the back to the buds and then put a knot in the cable near to that end. Slip knots are good for this because they’re easy to undo but will stay in place long enough.