Left justify, each on its own line:
To: From: Date: Subject:
This information tells who the memo goes to, who it's from, when the memo was written, and what the subject of the memo is. Bold each of these elements.
Write the memo in either first or third person. Either of these can be used, depending upon the purpose of the memo. Using first person can be an effective way for the head of the company to get on a more personal level with employees. For subjects that are more relaxed, such as announcing a company party of picnic, first person might be a better choice. A mandatory meeting might benefit from a third person point of view, written with a more formal tone.
Keep the memo as short as possible. Between three and five paragraphs, each with between three and five sentences, is appropriate for most memos. Memos are meant to be short reminders. Be direct in the body of the memo. Get right to the point. If there are any special instructions, try listing them as bullet points when possible. End the memo with contact information for those who may have further questions.