Don't use 2nd person.

Avoid using 2nd person ("you" or "your") in memos. Second person is only appropriate if you are intending to address each reader as an individual. For example, I use "you" a lot in this document because I really do mean to give advice to you, personally. However, I would never use it if I were writing to a group, or if I thought that the memo might circulate around an organization. In those situations, "you" would sound either overly informal or just plain goofy because readers would know that I was not writing the memo to them, personally.

Bad: "If you adopt the proposed carbon tax, your city would gain $100 million in revenue."

Problem: First, the "you" at the beginning: Does every reader get to decide about adopting the tax? Nope: even the mayor would need a vote from the city council. Second, the "your" in the second clause: Do you know that every reader regards the city in question as his or her city? What if the document is circulated to someone in a neighboring community?

Better: "If Los Angeles adopts the carbon tax, its tax revenue will increase by $100 million." Much clearer and more straightforward.

An additional reason to avoid second person is that "you" can be confusing when it's used as an informal alternative to "one", as in "when you subtract costs from revenue, profits are $100." The author doesn't mean the reader should literally do the subtraction; rather, she means "when one subtracts costs from revenue." No one really likes to use "one" so it's usually best to phrase things differently, such as: "Subtracting costs from revenue shows that profits are $200".

One final reason to avoid second person in a memo is that if it's not done well it can come across as condescending--as though you feel you're superior to the reader and are entitled to tell them what to think or do. This is a very tricky point. When you've done an analysis, you're likely to know more about the policy than the reader--in fact, that's your job. However, you need to get that across without sounding like you think you know more about everything. Second person makes that a lot harder and is best avoided.