It should go without saying that sunscreen is an absolutely essential part of long summer days spent in the sun. But as a quick glance at the drugstore shelves will tell you, they're not all made equal.

The trick is to know what's a marketing gimmick and what's worth buying. Arm yourself with this info, then pick up a bottle of whatever fits your bill and use it generously. Skin damage is no joke.

Don't Sweat the SPF

You know those eye-catching bottles that boast an SPF of 100+? Ignore them. Sun protection doesn't work as a linear scale, so the higher numbers aren't worth the cash. SPF 15, for example, filters out 93% of damaging rays, while SPF 30 filters out 97%, and SPF 50 filters out 98%.

If you have especially sensitive skin or are at risk for skin cancer, then by all means, go for a 50 or higher. But otherwise, you'll be all set so long as your sunscreen has an SPF in the double digits.

What's far, far more important is that you reapply every two hours or so. Yes, it's a pain, and it's easy to forget about when you're running around outside. But force yourself into the habit anyway. You'll avoid nasty sunburns and prevent rays sinking into your skin and causing long term damage.

Go Oil-Free

If you're prone to shiny, oily skin or acne breakouts, an oil-free formula is your best bet. Just like oil-free moisturizers, they'll leave less of a greasy shine so that your skin stays looking matte and fresh. That's especially good for whatever sunscreen you wear daily on your face, since a shining forehead that oily sunscreen causes can make you look sweaty and grimy.

(Oh, and if you don't already have a sunscreen you wear daily on your face, you should. Seriously. Blocking wrinkle-inducing UV rays is the number one way to keep your mug looking young and spry.)

An oil-free blend is less important for a full-body application, since a slight sheen to your legs, arms, and shoulders is no big deal. But it might be a good idea if you're prone to getting breakouts on your back or neck, since the oil-free formulas have less risk of clogging up your pores and causing acne.

Cover Your Bases With Broad Spectrum

There are two kinds of solar rays that can wreck your skin:

• UVB, which hit the more superficial layers and cause sunburn. And they're especially dangerous, since they're the prime suspect in causing skin cancer if you're not careful about protecting yourself.

• UVA, which are less intense but far more prevalent. These rays sink into your skin more deeply and, over time, cause wrinkles. These used to be considered relatively harmless, aside from the visible aging they cause, but scientists have lately been seeing that they can also contribute to (and, in some cases, even kick start) the development of skin cancer as well.

Don't waste your time with a half measure. To get proper coverage, look for formulas that specifically say that they're broad spectrum or UVA/B.

Look For the Right Active Ingredients

Broad spectrum coverage is good. But unlike UVB protection, which uses SPF as a benchmark, there's no easy way to quantify how safe you are from UVA. You can, though, look for specific active ingredients that are well tested in counteracting the rays. Keep an eye out for titanium dioxide or zinc oxide, which are minerals that reflect those rays away.

They're inexpensive to produce (meaning the resulting sunscreen is relatively cheap), they're strongly effective, and they've been successfully used in sunscreen for ages. The only downside is that they can make your sunscreen very chalky, leaving you with a pale white pallor, but most formulas have gotten good at eliminating that effect.

There are some newer options out there that use chemicals like avobenzone or ecamsule, but they're expensive and not worth branching out for.

Don't Worry About Waterproof

The FDA says that water resistant sunscreens have to be able to keep their SPF even after 40 minutes of swimming or sweating. If you're the kind of guy who likes to splash around or plays outdoor sports, investing in some might be a good idea.

40 minutes isn't that long, though, and it's likely to give you a false sense of security. You still need to reapply often, especially if you're in the water or working up a sweat for extended periods of time.

Another downside: it's tackier than normal sunscreens, making it a pain to apply. Stick with the normal stuff, and reapply whenever you're done swimming or playing.