State your conclusion first then back it up.
Do not make someone read through pages to find out where you are going. The reader may get lost or bored, or both.

Example of a good beginning: "This memo summarizes the evidence in favor of defendant Jones showing he did not cause the accident."

Example of a bad beginning: "Joe Jones was born in 1973 in Tulsa Oklahoma, and received his driver's license in New Jersey in 1999 after his family moved there."

In the bad example, we don't know why these facts matter or where they are leading.

Reason completely and explicitly.
Leaving gaps in your thought process, or in the information behind your thinking, invites misunderstanding because the reader will consciously or unconsciously fill in the gaps and may come to a conclusion or make an inference which you did not mean or intend. "Smith didn't witness the accident" may suggest he has no evidence to give. However, what if Smith attended a party where the driver allegedly got drunk? In that case he might have evidence to give. The complete thought might be: "Smith was not at the party nor did he witness the accident. It is therefore unlikely that he possesses any relevant evidence."

Don't assume the reader knows what you're thinking, or has the same information you do.
All relevant information should be explicitly set forth. Do not assume, for example, that the reader knows which party is insured by XYZ Insurance Co.

Write well.
Use the active voice
Use short sentences
Use plain words
Avoid ambiguity
Use technical terms correctly
Use proper grammar
Spell correctly
Use foreign language phrases correctly

The Ultimate Keys. Re-read, re-think and re-write.
The product will be vastly improved, no matter how good the initial draft.