How can you prepare turnips to make them less bitter? I peeled and quarted turnips, then roasted with carrots and onions in olive oil and maple syrup, but the turnips were really bitter. Is there a way to roast them (or even another cooking method) to remove that bitterness? How long should they be roasted? 

Or do I simply need to pick a better batch of turnips?
Yeah, maybe you just had some bitter turnips.  It sounds like you did all the right things to hide the bitterness.

If you want to try a different cooking method, I tend to like turnip boiled and mashed.  If the turnip is old (and likely bitter) you can add an apple.  I've also read that you can stir in baking soda after the turnips have boiled to remove the bitterness. You would then need to rinse thoroughly. I've never tried this however.