Here is what I do: I treat my neck, shoulders and chest as extensions of my face- I use a high SPF, high PPD moisturizing sunscreen on those areas, one that's formulated especially for faces. On my arms/legs/feet/hands/whathaveyou, I use another sunscreen formulated for the body, but I'm not so picky about it. I have a few brands lying around and generally they range from SPF 30-55, and PPD's of 10+.
While SPF is important to me (obviously I don't want to burn!), I am most mindful of getting a high PPD sunscreen. It doesn't matter how sunny it is, what time of year it is, how "safe" you think the place you're living is, UVA rays are strong all year and all day long!
londonkiwi brings up a good point about the lips- I always forget to reapply my lip balm when I go outside, I should be more careful!