Hey I'd just like to say the key to sunscreen is reapply, reapply, reapply! Also make sure that you get it on 45 minutes or so before heading out, and reapply every few hours.
I work for a company that makes sunscreens. So here is an interesting fact: In the US there is really no such thing as a SPF rating higher than 30. OTC protocalls only rate up to 30. Not to say that that bottle of 70+ won't protect you, but there is a good chance that it won't be much better than a 30. Also, I've learned that freezing and thawing a sunscreen can, in some formulas, drastically reduce its effectiveness! So keep it in your pocket, not in your car, you will also be more likely to reapply!
Enjoy the sun and corn everybody!