Let's take a look at an extended metaphor, shall we?
Pick a subject, something you're dealing with or have dealth with recently. Boredom, loneliness, angst, depression, anger, fear. Something that ails you.
Now choose a gneral metaphor to go with that issue. Water, fire, a train, a bus station.
Write a poem exploring this metaphor, but dissect your metaphor into several pieces. Start each stanza, or every few stanzas (or line breaks, or something... depending on how you write) with a new iteration of this metaphor. Find new wayts to use it and try to get beyond the obvious, or your first thought. Mine the idea for everything you can.
If you go for water, you can break it down into a lake, ducks on the lake, fish in the lake... the drain pipes carrying the water to the ocean. If it's fire, make it kerosene, napalm and ash. If it's war, choose different wars, or battles, or even the parts of a war, the front line, the infantrymen and the radio chatter.
Sketch out scenes with each stanza exploring these ideas. Vary it up. Carry your ideas somehwere new. Surprise yourself.
Take a look at Dear Someone by Deborah Landau.