DD's neck was flexed at birth (resulted in needing an emergency CS) and she always favoured her left side straight after she was born.

She even look like a banana, lying there! Her head would be on the left, neck curved to the left, torso straight and then hips curving to the left too.

We saw a osteopath and had a number of sessions (5 or 6, all up, I think - 4 in two weeks to begin with) to help with her torticollis, and we noticed a good difference in one session, and by the end of the 2nd week, we noticed a BIG difference. Much more relaxed in her body, and could start turning her head to the right. The osteo said she had it pretty bad to begin with - very tense and contorted in her body. We started her treatment with the osteo when she was about 4-5 weeks old. Although the younger the better, my osteo told me.

I encouraged her to then strengthen the muscles on the right side of her neck by holding her on my other side so she could look out at the world with her head on the right (both cradling her and holding her upright), and as someone said, toys on her right side, and also a little dangly music box we had attached to the side of her crib. Anything to encourage her to start turning and using the muscles on the other side.

It took a while, but she started using both sides equally. She has a slight flat spot still, but that's difficult to get rid of completely until she's rolling/moving on her own as she now loves to suck her left thumb when sleeping - so her head is still on the left!

Tummy time is really good too, and putting toys whilst she was on her tummy on right side. Also helps with flat spots.