How to Write a Metaphor

I like Wikihow, and this is a good article on how to write a metaphor. (It touches on similes, too.)

"Metaphors are the cold knife in your side. They are the speed bumps that keep you from picking up writing momentum. They are the hidden monster lurking in the closet of ... of ... oh, darn it. Metaphors are tough, no doubt about it. But approached methodically, metaphors can be the spice in the cuisine that is your written work..."

Their best tip? Practice! :)