Forming a habit is a good thing to do. It's hard work at first but then whatever it is happens like magic and something is missing if you don't do it. A great way to form a habit is to make a resolution. "Do X for Y weeks."
Resolving to have a good day might be too big a step to take, but resolving to think of two things that made today good is an achievable goal. Perhaps it would help if to taping a note to a bathroom mirror or setting an alarm on a calendar to "Think of a good thing".
Even good habits; habits that make you feel good,can fall by the way-side. Sometimes, having accountability can help too. Make sure when you resolve to do something to tell people what you are doing and why . . . because sometimes we all need a gentle reminder that "Today is a good day".