Kate Middleton attends the U.K. premiere of “War Horse” on the eve of her 30th birthday — and in style, as usual. (Ian Gavan/Reuters)

So on her 30th birthday, we’ll forgo composing sage advice for this new decade (she doesn’t seem to need our help) and instead reflect on some of the fashion advice we’ve garnered from the stylish princess this year.

1. Simple is chic

If we’ve learned anything from the gorgeous duchess and her perfectly bouncy hair, it’s the virtue of minimalism. Kate’s best looks are most often composed of a well-fitting sheath dress, her infamous L.K. Bennett nude pumps, maybe a pair of diamond earrings and a Mulberry bag. The recipe is simple, and it works.

2. Tailoring

One of the reasons this formula works so well for Kate is that her dresses are always impeccably tailored. They are form-fitted without being tight and showcase her figure without being the least bit vulgar.

3. Designers matter, but street style has its place

Kate has given Americans an appreciation for Reiss, Issa and Erdem. And has furthered our love for Sarah Burton and all things McQueen. But the duchess’s ability to go from a floor-length beaded Jenny Packham to a geo-printed Zara dress has endeared her to us commoners. If a future queen can wear Zara with pride (the day after her wedding, no less), so, too, can we.

View photo gallery: A look back at the best outfits worn by Catherine, the Duchess of Cambridge, in honor of her 30th birthday.

4. Sheer panty hose

Few would have imagined nude panty hose would ever reenter our closets. For years, they were what old aunts with open-toed shoes would opt for. But covering Kate’s lean legs has convinced us there is life in the sheer stockings yet.

5. Headwear

Kate has also reminded Americans on the importance of properly matching headware. We have long been behind the curve on this trend, forgoing numerous opportunities to accessorize with an eye-catching fascinator. Thankfully, Kate and sister Pippa have shown the necessity of these garments for all formal, outdoor occasions (anything with horses, gardens, tea).

More on Kate’s style:

Kate Middleton: All of her best outfits for 2011

Top 5: Kate’s best looks from her North American tour

Kate masters the outfit change

Royal weddings in 2011