See how respected poets have used alliteration. For alliteration, check out the poems of Gerard Manley Hopkins, especially "The Windhover." In that poem, he says, "I caught this morning morning's minion, kingdom of daylight's dauphin dapple-dawn-drawn falcon in his riding." That one pushes the edge of the envelope of how much alliteration you can get away with in a poem, but Hopkins made it work. Note that even nursery rhymes have clever alliteration: "Goosey goosey gander," and, "Peter, Peter pumpkin eater."
Use alliteration to spice up a poem. Poetry needn't be somber and introspective. You can have fun with it, and when you do, you can give your readers enjoyment. Shakespeare seemed to relish in delivering unique alliteration to liven up his poetic dialogue. When Hamlet says of the actor who was weeping about Hecuba, "What's Hecuba to him, or he to Hecuba?" that's Shakespeare having fun. Shakespeare's work is full of alliteration: "bare bodkin," "self-slaughter," and "band of brothers." You can do the same with your poems.
Research how heralded poets have used metaphors. A good example is Carl Sandburg's poem "Fog." The entire poem is a metaphor.
Write down good metaphors that you come across when reading poetry. This can help you come up with ideas for creating your own sterling metaphors.
Make sure that any metaphors you use in your poems are fresh. Avoid metaphors that have been used so often that they're clichés, such as referring to the world of work as "the rat race." Again, Shakespeare is the master of unique metaphors, such as, "When we have shuffled off this mortal coil," which means, when we have died.