Humorous Asides  

• Oral or written humor requires two people: the jester and the straightperson.
• If your story is primarily light-hearted, be extra careful not to accidentally insert lines that aren't supposed to be funny, but may read that way.
• Don't use humor just for the heck of it. If a character is not portrayed as a wit start to finish, the sudden shift to uproarious rake won't fly.
• People can't laugh, snicker, chuckle etc. and speak, simultaneously. Separate the action from the dialogue. Also, if you're compelled to add qualifiers such as "He said, humorously," "She added, wittily," "He drawled, sarcastically," it's possible you're not getting that point across within the dialogue. Show those attributions by word choice, rather than tell the reader (after the fact) how the lines were delivered.
• Watch using current slang, news topic references, cliches, etc. The first two date your work; the latter can make for dull, anticipated dialogue.
• Similes and metaphors are not just descriptive tools for narrative passages--we tend to use them when we speak, too. Just don't overdo it.
• Timing is everything. Have a friend read your humorous passages aloud to you. If he/she doesn't catch the cadence you tried to effect, it's likely no one else will either. Humor has rhythm; punctuate accordingly.
• It's tougher to make a reader laugh, than cry. But you can do it. Like all fiction techniques, it just takes practice. The deeper you're inside your funny character's skin, the more naturally those on-target reactions/remarks will occur to you.