Humor

We all love humor.  When it works it is stimulating and liberating at once.  We love to see it.  We love to read it.  We love to live it.  Unfortunately, we don't always love to write it.

When you think about it humor is perhaps the one thing you can write that can generating a profound physical reaction in the reader.  Keeping that in mind it is not a great leap to appreciate that humor can be a potent tool for resetting story tension to allow for another scale-up.  Humor can also be a useful tool for clarifying characters.  In fact, that leads to a few analytical comments about humor and how to do it.
 

Humor usually has a target.  Sad but true.  Humor picks on someone or something. As such it can help define characters.
Humor sets up a false expectation.  Humor is a reflection of societal values and tendencies.  Capitalize upon expectations.
Humor is an unexpected yet plausible twist.  Sound familiar?  That's what much of your story writing is.
 

A strategic approach to humor is to decide where in your story you need it.  Then develop a false expectation based upon clues you have secreted throughout the earlier scenes.  Finally, twist the outcome so that a sub-villain is made into a buffoon. Don't use it on your antagonist.  They must be monumental-- at least until the end.

A non strategic approach to humor is to use your journals or notebook to record the funny things you see and experience through out your day.  Translate these events into your story when you can.  Later test them on a sample reader to be sure that they still hold up.