I use sunscreen all the time when I'm out in the field. I usually use 45+, and on a good day I actually reapply the stuff every couple of hours. I tell people that I'm so diligent because I'm concerned about my skin's health, but the real reason is that I detest farmer tans.
Since I wear t-shirts nonstop in the summer if I'm doing fieldwork, when I have the opportunity to not wear one, I do. So you'll usually find me in something sleeveless or with a scoop neck. I look ridiculous with a farmer tan and a frilly little camisole.
So I was outside a couple days ago, wearing a t-shirt, and I did in fact apply a lot of sunscreen. However, I apparently missed the sides of my neck and a 1-inch strip right under the edge of my sleeves (both arms). I now have the ultimate farmer tan, and I'm going to an outdoor semiformal event (think sundress) in a couple of days. Grr.