Lots and lots of hairclips.
Granted, you'll probably find them all over the house and on the side of the bath but it'll save her the trouble of constantly tucking her mane behind her ears...
Evidence for our suggestion? The many many many many many pictures of the Duchess fiddling with her locks which we have condensed into a handy GIF so you don't even have to trouble yourself with clicking on a gallery. That's how nice we are!