Decide what you want to say. Start simply by jotting down, in basic terms, what you are trying to communicate. For example, if you're trying to write a congratulatory message to someone who just graduated, you could jot down, "congratulations on graduating."
Employ figurative language. Add a simile or metaphor to your message, setting up a comparison between two seemingly dissimilar things. For example, you could say, "Your success is like the climbing of a treacherous mountain." Or add some sound effects by including a onomatopoeia, or words that represents a sound such as "bam" or "bang."
Include strong adjectives. Though you don't want to compose a message that is so rife with adjectives that it becomes unnecessarily complex, adding a few, strong well-selected adjectives could greatly strengthen your message.
Add allusions. Make reference to master works through allusion to make your message more creative. For example, if composing a message to someone who is a major fan of Jane Austen, include an allusion to a work by this author, such as referencing to Mr. Darcy or Pemberly.
Keep it concise. Don't allow your eagerness to make your message creative lead you to compose a message that is overly complex. Instead of filling your message with all of these potentially creative elements, pick and choose to keep your message a reasonable length.