Don't be the person who writes a memo for everything. There is still such a thing as a phone call, as well as bumping into someone in a hallway, or just plain going to speak to someone in his or her office.
What They Need to Know
Once you identify the who and why of a memo, you need to focus on the what. Memos by nature contain very specific and focused information, so don't ramble. If you are bringing attention to something then it's important to include a brief background, specific points that you are making about the issue, and finally what it is you want back from the reader. In other words, include anything the reader would need to make an informed decision.
For example, let's say you're a supervisor and you are having a disciplinary problem with one of your subordinates. You formulate a memo that asks to invoke a formal disciplinary process. In this example, you must include the background that led to this point, what you have already done in order to try to correct the problem, disciplinary history, what policy violations are involved, and what it is you need back from the reader to continue. If there is enough information to suggest a policy violation, then the last item covered should be seeking permission to go forward with an internal affairs investigation.
On the other hand, if you are not asking something but are giving instructions to take some type of action, you still mention the background, but you then add where you are with the issue, and state how you want it handled. With an action memo, you need to include all the information necessary for the reader to become successful in completing the task. This means including the parameters, limitations, and the date when it needs to be completed. You can expect clarifying questions; minimize them by being as thorough as possible. Your failure to describe what you want done will result in the recipient's failure to get it done.