I'm a generally upbeat and positive person. I went through a bout of depression some years ago related to guilt, denial, and repression of being transgender, but I've grown into it. I am transgender, I own it, I enjoy it.
Introspection and self-analysis are two of the tools in my Happiness Toolbox. Analyzing why I feel negative emotions helps me to come to terms with them, to understand and integrate them. I try not to toss the problem aside, that just leads to bigger problems later. I try to be rational even though I am fairly emotionally driven.
I could go into a recent f'rinstance, but that would take far too long for anyone to read. I question the answers, too. Sure, this can go into an infinite regression, but once I go deep enough, I own that emotion, the reasons behind the emotion, the reasons behind the reasons, etc. ad nauseum. I have boxes and boxes of shorthand reasons for everyday problems that make the process a lot faster. I try not to delude myself and succeed most times.
Another tool: doing things that make me feel good. Like, duh?
When I can help someone help themselves through a problem, I feel good. When I can make others laugh, I feel good. When I make something others enjoy, I feel good. When I reach a goal I set for myself, I feel good. When I learn and grok something new, I feel good. Do good, be good, feel good.
Yet another: I try to find beauty in all things. There is beauty in a child crying. Pure, honest emotions are beautiful. If I open myself up and look hard enough, I can usually find something beautiful about nearly anything.
One more for now: I sing (off key), whistle (very well), and hum quite a bit. It's hard to be in bad mood when you're singing an upbeat tune. Christmas carols are my worst vice.
I try to look at the positive side of any situation. My girlfriend just broke up with me after admitting that she was seeing someone else when we had agreed to a monogamous relationship. This caused a lot less pain and problems than it would have if we were married or living together. We had some great times together. I learned about myself and others. I'll have more great times later with someone else. I will be able to save more money now. I feel free to flirt voraciously again and I love to flirt. And more.
There are many more tools in the box, but these stand out the most in my mind.
When I start feeling unhappy about something, I do one or more of the things above and they usually keep me upbeat and positive.