A friend of mine once heard a speaker share the following formula:
When you tell a joke about somebody, it's 40% joke, and 60% true.
What he meant was, if you dissect a joke, 60% of it is what you truly think.
My friend passed this formula on to me and several friends, and we had a good laugh about it. Any time one of us made a joke about someone, someone else would invoke the principle by yelling, "Sixty forty!" and we would laugh at it all over again.
But honestly, I think this formula has some merit. I don't know that the ratio is 60/40, but when you really think about it, don't most jokes contain some truth? That's usually what makes them funny, because we recognize the truth in them. This isn't usually a problem unless we are joking about our friends, spouse, coworkers, children, etc. In that case, 60/40 isn't such a good thing. The 60/40 phenomenon may be a bit cheesy, but it's also real - you can end up hurting someone you love without really meaning to.
So the next time you want to make a joke about someone, think about 60/40. If part of the joke were true, would it hurt the person you're joking about? The joke may be funny, but it probably isn't worth hurting the feelings of someone you love.