I just try to be aware that things will eventually get better, even if it's a long time from now or even if I don't know when. Life goes through cycles of good and bad, and it's easy to lose sight of that. I know I often have a hard time keeping this in mind, but I find that when I do tell myself "this won't last forever", I often feel better, even if only marginally.

As rcat said, distractions can be helpful too. I've been very nervous about one particular thing coming up in a couple of days, and keeping myself busy has been helping me to overcome that. A lot of the time I fall into distracting myself with nothing but TV though, which I feel is probably unhealthy. :/