One or two outstanding skills or personal characteristics, again make them relevant to the job you are applying for. A reference to your values, commitment, or philosophy if appropriate and relevant. Indicate experience related to the job you are applying for. Ask your reference if they will recommned you for the job. If you don't ask you don't get.
Every word counts, so keep it short and simple.
Avoid abbreviations or jargon. Keep it neat, typewritten and use quality paper. Keep it short - one or two pages. Use dynamic verbs to describe skills and experience. Have others check it for grammatical errors.