I'll second the suggestion to read poetry -- but also try your hand at writing it. If you've never done it before, it can be extremely awkward at first, but it's worth the effort (and remember, you never have to show them to anyone). Write a poem about a single emotion, or a scene, or an event you've experienced, and the intensity of the short-form will invite you to draw on other emotions, scenes, or events to help convey its meaning.
I've found that my similes and metaphors come from my passion for wild places -- which is convenient since I mostly write Fantasy with a lot of traveling.
Whether I'm hiking or camping, or wandering a beach, those places remain vivid to me even after I've returned to my desk. I can describe them in a sort of free-form narration -- and that's often what springs to mind as a simile or metaphor when I'm writing.
What are your passions? What places are evocative for you? Where do those things intersect with your work? Find out, and then spend time wandering; go more than once, and at various times: at dawn or dusk, or in the dark, when it's crowded, deserted, too cold for comfort, or too hot. You don't have to take notes, or memorize what you see -- your subconscious (your muse) will remember it for you.
As has been said, metaphors and similes aren't necessary for good writing. But if you enjoy them, there's no harm in investing a little time loading up on material to work with.