Train yourself to recognize metaphors so that speaking in metaphors will come more easily. When you read, watch television or see a movie, look for images and listen for dialogue that symbolically imply more than their literal meanings. For example, if a movie character is curled into the fetal position, the position is a metaphor for vulnerability.
Don't use the words "like" or "as," which create similes rather than a metaphors. A metaphor compares one thing to another without using the words "like" or "as." For example, if you say that someone is a rock, "rock" is a metaphor that characterizes the person as strong and steady.
Use metaphors that will be meaningful to your listener when possible. For example, if you have a friend named Cindy who makes everyone laugh, and you meet a new friend who is also very funny, you might say, "She's a Cindy!" Anyone who knows Cindy will know exactly what you mean.
Avoid mixing metaphors, which will cloud your meaning, although it may make people laugh. For example, referring to increasing trouble and confusing "out of the frying pan, into the fire" with "tempest in a teapot," you might say, "out of the frying pan, into the teapot."
Don't explain your metaphors. That ruins the effect. Either your listener gets it or she does not. Even if the listener does not get it right away, later she might, so let it rest.