While the debate between pro-earbuds vs. pro-headphones camps continues on, I'm of the opinion you should have both. I wear earbuds exclusively while running and working out, but there's always the annoying pendulum swing when popped out of the ears. Crowdsourced design factory, Quirky, comes around with a proper solution: Props.
The adjustable length polypropylene and nylon cord tethers earbuds to the neck area, keeping our earbud cord in proper position both when in or popped out of your ear canals. Not bad for a $5.99 accessory.
Or maybe you just need larger earbuds?
Of course, there other earbud tangle preventive measures, some retail, some free, others DIY: