LO shifts so much in her crib!

Please tell me I'm not the only one!  I put my 6-month old daughter in her crib the "normal" lengthwise position every night, and within 15 minutes she has completely shifted 90 degrees and is sleeping with her head by the "headboard" and her feet by the "footboard" side of the crib.  She is barely 1-inch shorter than that length of the crib at this point, and will often wake herself up because she's hitting her head on the top wall.  How can I get her to stop shifting??  Or should I resort to bumpers?