Write about your day. Concentrate on observation and description, not feeling. For example, the ordinary reaction is to write “I received a letter today and it made me happy.” Instead, describe the size of the envelope, the quality of the paper and what the stamps looked like.
Keep your diary without using the verb to be. Forms of to be don’t create any vivid images. By avoiding its use, you get into the habit of choosing more interesting verbs. You’ll also be more accurate. Write “John makes me laugh” instead of “John is a funny guy.”