Keep in mind as soon as you get your invitation:
Even though you aren't the bride, it's important to remember you'll still be photographed (and photographed a lot, if you're a bridesmaid) throughout the day. Stick to a solid skincare regimen to keep any breakouts at bay and even out your skin texture. If you highlight or color your hair, considering having it touched up a week or two before the wedding. A fresh mani/pedi is another (relatively inexpensive way) to freshen up your look. If you get spray tans routinely, you should schedule a touch up before the wedding, so your skin tone is even. Also, avoid experimenting with any new, more invasive beauty treatments with unpredictable recovery times (think: chemical peels, injectables and microblading) right before the big day.
Day-of hair and makeup:
A great rule of thumb for wedding day hair and makeup is: Don't overdo it. Pinterest has a lot of really pretty options and is a great place for pre-wedding beauty research, but it's easy to get "pin happy" and end up with a super over-the-top look that doesn't necessarily reflect your personal style or feels out of place. No matter the hair or makeup you choose, you should aim to look like the best version of yourself.
The light is harsher and the air is more humid in summer, so I'd avoid any super thick serums or creams that will make your makeup crease faster and make your skin appear greasy. In that same vein, it's a good idea to opt for a more natural makeup look in general for summer weddings—especially if you're attending outdoor or beachside nuptials. Intense smoky eyes, heavy contouring and lots of highlighter just seem out of place in those settings. Though it's counterintuitive, I'd also skip foundation and moisturizer with SPF, since they can wash your complexion out in photos.
What to pack in your clutch:
How to refresh mid-wedding:
Dry shampoo is a fast way to perk up any limp locks after a night of dancing or sweating. Oil blotting sheets are also a must if you have oily or combination skin—they're a much better alternative than just adding more powder to your face to tamp down shine, which can start to look caked on after a couple of touchups. No matter your skin type, a few spritzes of a good face mist (I love Caudalie's Beauty Elixir) can instantly refresh your skin, especially if you're in a very humid climate. If you start to look tired or washed out at the end of the night, dust a little blush over the apples of your cheeks and reapply your lip color—it's an instant pick me up.