Sunscreen is a tricky one. Firstly, something many people don't realise is that all sunscreens have two different ratings, one for UV-A and one for UV-B (there is also UV-C, which refers to even shorter wavelengths, but it is mostly absorbed in the atmosphere and not considerd for sunscreen). UV-A has longer wavelengths, and therefore less energy, than UV-B, however, it penetrates skin easily. It does not cause sunburn, but is responsible for most of the damage. UV-B is higher energy, but does not penetrate the skin effectively. It is UV-B that is responsible for sunburn, and so this is easiest to measure protection.
The most basic way of measure the protection factor of sunscreen is to apply it to a volunteer and see how long they take to burn. It is also possible to make similar measurements with a spectrometer, although I'm not sure if this is particularly common. The important point is that this only measures protection in the UV-B range, since the effects of UV-A are essentially invisible. Since it is this measurement that is displayed as the SPF on sunscreen, you don't really know if you're actually preventing damage.
Because of this, there are various systems to rate UV-A protection, but they are not particularly widespread, and some, the Japanese system for example, are of questionable use. In Britain, there is a simple star system, from one to five stars, that describes the ratio of UV-A to UV-B protection and is now used by the majority of producers. However, while this allows you to rate one sunscreen against another, the actual protection afforded is harder to find out. I don't know if there is any UV-A rating at all done in the US.
What all this means is that it is entirely possible to have sunscreen with a very high SPF that actually provides very little protection from damage, it simply prevents the cosmetic effects. Rather than just using sunscreen and assuming you're safe, it's important to check that it's actually effective against UV-A, otherwise it can be worse than not wearing anything, since you will tend to spend longer in the Sun.
And this comes to the other big problem with sunscreen. If you wear it, you're likely to stay in the Sun longer. You can use the best sunscreen in the world, but if you stay in the Sun ten times longer, you'll still end up worse off than wearing nothing. This is an argument made for a lot of safety equipment like bike helmets and seatbelts, however,
studies show that melanoma, and death rates from melanoma, really do increase in areas with more sunscreen use.
Which of course brings up the final problem. Is rubbing funny chemicals into your skin good for you? There is certainly a problem with allergic reactions and sensitive skin. Although there are hypoallergenic sunscreens, some people can't use any. And of course, since increased sunscreen use correlates with increased cancer, cause and effect can't be ignored. As far as I am aware, there is no good evidence that anything in sunscreen is in the least harmful, despite plenty of speculation from the usual suspects. However, widespread sunscreen use is a fairly recent phenomenon, and the field is devolping constantly with new substances being used, so while it is probably completely safe, the possibility of adverse effects can't be ignored.
As Devnull says, wear a top and a hat. You can still burn through clothes, especially light, summery ones, but a physical barrier will always be better than a chemical one. However, you will find it harder to ask for massages.
