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.