Know your characters inside and out. Theater actors often invent a detailed back story for their characters, so that they more deeply understand the characters' motivations onstage. Think of your characters as real people and spend some time getting to know them before you begin to write. Where did they grow up? What were they like in junior high? What are they afraid of? Even though this information might not make it into your final draft, knowing your characters well will help you describe them in colorful, realistic ways.
Use exciting language. Invest in a quality thesaurus or find one online. If you find yourself using the same adjectives repeatedly, seek out new and interesting words to try. For example, instead of "funny," use "capricious," "whimsical" or "facetious." Each of these words has a slightly different meaning that will texturize your writing and keep your reader engaged.
Be specific. It's tempting to write about vague, abstract concepts like "joy" and "pain," but without concrete details to back those ideas up, your reader may lose interest. Instead of writing simply about joy, write about the people, places, smells, tastes and sounds that bring you joy. Keep your writing exciting by infusing it with plenty of precise and sensory description.
Have a friend check over your writing when you're finished. It's always a good idea to get a second opinion, and they'll be able to tell you what's working in your writing and what may need further work. Never take constructive criticism personally and don't be afraid to revise your writing.