Start an Inside Joke

Starting an inside joke can be fun and relatively easy. All you need are some close friends!

Steps

Have a close group of friends. The first step to a successful inside joke is good friends; people you can pretty much say anything to.
Keep it quiet. An inside joke is something that only a select group of people know about, so don't go telling everyone and their mother about it.
Notice things around you, because that's how they usually start.What did that kid just say? What's wrong with him? Or maybe, your friend recently was give her uncle's ipod, only to find a very odd song on it. Think like that.
If anything hilarious happens, such as your friend embarrassing his/herself in front of his/her crush by falling, you could sing a song related that to falling whenever s/he's around.

Sometimes inside jokes are better if they are small things that can be easily brought up by a single word or phrase.
It's fun to bring up the inside joke in 'mixed company'--i.e., other people besides your group are around. Just don't tell them when they ask what's so funny.
The smaller the group, the better.
A good inside joke has a cryptic or mundane sounding reminder.
Keep your joke funny by refreshing it every so often, don't just let it be about one thing every time, spice it up!

Don't go overboard laughing at it because inside jokes can be killed that way.
Try not to create hurtful jokes about people outside the group, or you could end up hurting someone's feelings badly.
Don't try to make a joke out of something really embarrassing or painful, unless the 'victim' thinks it's funny as well.