In my current world there are no open bodies of water, so I can't really use any metaphors to do with waves, or the sea, or the sound of the sea. So in a crowd situation I couldn't say "the sound roared over her like a wave". I might try instead "the sound roared over her like rocks spilling down a slope".
Use the setting of your story to influence the metaphors you use. If your story is by the beach, don't use "as cold as snow", try "as cold as the deepest sea". Also think about the images you might want to bring into your story. If your story is in a city, you might want to compare it to the tall trees of a forest, or to the loneliness of a desert, or the swarming life of a swamp.
Another good tip for practicing your metaphors is to read (or write) poetry. Poetry is filled with strong and unusual metaphors, something that can be vital in your prose as well.
What do you do when you need to think of new metaphors for your work?