I've got a song that I hum when things get difficult; when I get tired on a trail, frustrated with whatever, or just when I'm thinking too much. I'm not sure when I started, but I think it was some time during college or right after. It's a song by the Duhks, Down To The River; it's slightly folksy, a lotta bit of Cajun, and a little bit Irish jig. I imagine that if I ever endure something extremely difficult, like having a limb cut off while awake or childbirth I'll be singing it at the top of my lungs.
In case you'd like to get the catchy little tune stuck in your head, too!