Er, what are they again?
Friend Wiki states: A metaphor is a figure of speech that constructs an analogy between two things or ideas; the analogy is conveyed by the use of a metaphorical word in place of some other word.
Each metaphor gives a clear image of what the writer wants to get across. It should allude to atmosphere, mood, emotion or characterize. In the case above, LOVE is not a passionate teenage embrace (volatile, emotional), it is not the familiar touch of age-spotted hands (enduring, comforting). It is the teddy bear clutched in a sleeping child's arms--innocent, unconditional, protective, beautiful. The reader experiences LOVE exactly as we want them to. This is is why metaphor usage is so powerful. The analogy you choose doesn't only send an instant picture to the reader, it can also show them what you want them to FEEL.
So how do we build strong metaphors?
1) Write down the thing you want to create a metaphor for. It might be an object, person, emotion, a descriptive element, concept or physical action. IE: Winter
2) Write down several descriptive details/attributes/free associations about it. IE: cold, death, frozen, snow, ice, white, blanketing, clean, fresh, frost, blizzard, biting wind, renewal, isolation
3) Think about the emotion/atmosphere angle you want to convey in the scene and the meaning you want to get across. Light and whimsical? Dark and depressing? Symbolic? Humorous?
4) Create a list of possible comparables that have #2 in common while keeping #3 in mind. If you need to, start with a cliche to get your brain going, and then branch out into fresher territory. IE: # 3 Dark & depressing angle
...a linen shroud covering an earthy corpse (#2 the look of snow)
...the lonely howl of a wolf, separated from his mate (#2 isolation)
...the final icy exhale of a man on his deathbed (#2 death, endings)
Each of these brings an image to mind that reinforces a dark aspect of winter: death, being alone, finality. Metaphors often work best as a punchline of a descriptive passage, the final thought that summarizes the whole.
5) Choose the one that fits what you want to get across the most.
Metaphors are not only useful to show emotion and atmosphere, they can also work well when you need to describe something yet be economical with words. Equating a dance partner to a headless chicken in his death throws is often more effective more than a paragraph detailing his twitchy dance moves.
Like all things, metaphors should be used in moderation. But if you're struggling with how to show, want to get more meaning out of your description or want to add a level of sophistication to your writing style, the big M is your friend. :)