I can't remember who gave me the idea, but I have a small, special notebook and a special pen by my bed. Every night I write the date followed by three good things that happened that day. They can be big or small things, e.g. doing some art, my son telling me he loves me, completing a chore that I'd been putting off, or just having a nice chat to someone.
I keep it to three things each day, so I have a lovely record of strictly positive thoughts and feelings. Looking back I can see that every day is a good day if you look at it in the right way. Because of this I find myself noticing the good things that happen during the day more and more.
This is a quote someone posted on facebook which helps me: "On particularly rough days when I'm sure I can't endure, I like to remind myself that my track record for getting through bad days so far is 100%, and that's pretty good".