It's that time of the year again. Time to pack up the cashmere, boots, and scarves, and bust out the bathing suits, maxi-dresses, and flip-flops. But, shouldn't your skin care routine get the same kind of out-with-the-old, in-with-the-new treatment, too? We think so. That's why we spoke to Heidi Waldorf, MD, director of the Laser and Cosmetic Dermatology at the Mount Sinai Medical Center in New York City. Here she shares the scoop on how to clean out your shelves and shower and stock up on the skin care items you need now.

Before deciding which products you should be using now, you need to weed out the items on your shelves that are past their prime. Look at all of your bottles, tubes, and boxes (yeah, even those unopened skin care goods that you haven't even gotten around to using). If they have an expiration date that's more than three months in the past, it's time to toss them. "At that point the active ingredients will be less effective and the preservatives may no longer be protecting against contamination," says Waldorf.

If your products don't boast a date and you've had them for more than three to six months, it's likely time to let them go, too. And, "throw away any product that has a strange smell or a marked color change," says Waldorf.

The medicine cabinet isn't the only place that needs a clean out, so be sure to check your shower for expired products, then ditch those and pack away the rich oils and body butters you may no longer need now that the weather is heating up. If you've been using soap and following up with lotion or cream that seems too heavy for the season, change things up. Swap your basic soap for a creamy body wash that will add the moisture your body needs without leaving it slippery, like a lotion might when worn in hot weather. Try Dove VisibleCare Softening Crème Body Wash, $8.29.

If the products that remain in your cabinet or shower are heavier and more cooler-weather specific, don't feel like you have to go cold turkey with them, says Waldorf. You can continue to use these products while you transition into your lighter weight summer items. Use the heavier stuff at night only, or go back and forth from one moisturizer (or eye cream, cleanser, or body lotion) to another until you've finished up the goods you spent precious cash on.

If there's one product you must add to your warm-weather beauty routine, it's sunscreen. Waldorf recommends using one that provides both UVA and UVB protection. Look for ingredients like "titanium dioxide, zinc oxide, avobenzone, or mexoryl," she says. If classic sunscreen lotions tend to irritate your skin, reach for a mineral powder sunscreen instead.