If it's a specific incident, or a general collection of incidents that I can put my finger on (a.k.a. "I'm having a rough day") I distract myself by doing something nice for me - going out for a nice lunch, buying something I've been meaning to get for myself but keep putting off, make something nice for dinner, etc.
If it's just a general feeling of frustration or stress, and I can't link it to anything, I'll try to spend some time by myself to reflect on things, think about the things I am grateful for, etc. If I still can't place what's bothering me, the things on SoaringSpirit's list are great!
Except exercise, exercise always seems too healthy when I'm in a blue funk. I haven't been able to do that yet, although I'm sure a good walk (or stomp!) around outside would make me feel better, too.