Check for books at your local library for the format. Remember to include the who, what, where, and when in the first paragraph. This may be all that gets printed. Keep a copy of any press release you send as well as a list of contacts to which you send them. You may not even get noticed the first time.
Perseverance is the key to getting publicity.
Make a publicity calendar and plan something every quarter, or more often if you can manage it. Your potential market needs to know who you are and what you do before they can even consider using your services. Good publicity can help you achieve that name recognition, as well as credibility, with your target market.