Email is now the medium of choice for most business decisions. Unfortunately, most people have only the vaguest idea of how to write an email that actually persuades somebody to make a favorable decision.
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Here are the five key rules:
1. Make the subject line revelant to the recipient. Even if you're emailing somebody who generally reads your emails (like your assistant), the subject line "pre-frames" the email so that the decision-maker sorts it into the "something worth paying attention to" bucket or the "nice to know but not important" bucket. (Even worse: the "Why did this idiot send me this?" bucket.)
2. Frame the problem/solution with timeliness. Every decision is a solution to a problem, and some problems are automatically more important than others. To give your problem (and solution) priority, communicate it within the context of what's likely to be on the decision-maker's mind.
Has your firm just lost a big customer? Build your message around preventing further defections. A competitor just launched winning product? Build it around leapfrogging the competition.
3. Communicate in language the decision-maker values. Decision-makers inevitably see any idea from their own perspective and experience. Because of this, your problem/solution is more likely to be accepted if it's expressed in terms that address the practical business concerns of the individual decision-maker.
For example: If you're talking to a CIO, you emphasize the technical bells and whistles. If you're talking to a CFO, you emphasize cost control.
4. Reduce or eliminate downside risks. Even the boldest decision-makers want their bottoms covered, so anticipate possible problems and objections--and be ready with a response. Think through possible weaknesses or objections and answer the most important of those objections in the email itself. This creates momentum that allows you to move to the all-important close (which comes next).
5. Ask for the next step. If you've done all of the above, it's time to clinch the deal. Ask a final question or request that propels the decision forward.
Need an example? No problem.
First, here's the wrong way to do it:
To: Fred CEO
From: Joe HR
Subject: Fitness and Productivity 
I recently read the national "Fitness & Productivity Report" based on a survey that our company, and many others, participated in, and it included lots of interesting information. Many companies cited work group physical fitness as extremely important, but very few said their employees actually demonstrate this! In fact, they identify physical fitness as an undervalued competitive asset, but they didn't have a plan for improvement in this area. I agree with the report's point that physical fitness is strongly linked to corporate and individual economic and personal success, in short, our success as a nation.  I feel that if we do not address the issue of physical fitness as it enhances workplace productivity, we will be left behind. Therefore, I'm thinking that we should consider converting the secondary conference room (which as you know is seldom used) into a gym.  Your leadership on this issue would be much appreciated.
 Irrelevant. Unless the decision-maker is a fitness buff, he'll probably just hit the delete key.
 Meaningless. The decision-maker has no idea why he should care about any of this.
 Irrelevant. The decision-maker doesn't care about your opinions. And this is a business memo, not a political stump speech.
 Ineffective. Finally, here's the purpose of the email, buried where the decision-maker is unlikely to find it.
 Annoying. Just what every CEO needs: another action item. You want a decision, not to upwardly delegate the execution of your solution.
I might also note that the entire email above is presented as sold block of wordy sentences and biz-blab.
By contrast, here's the right way:
To: Fred CEO
From: Joe HR
Subject: How we can easily decrease absenteeism.
As you know, sick days are clobbering our productivity. During flu season, for example, we're often so short-handed that we can't answer customer calls!  We could hire to backfill, but there's a most cost-effective solution: convert the secondary conference room to an in-house gym. 
We recently participated in a national survey entitled "Fitness & Business Productivity Report." The final report, which I have on my desk, points out that:
-- Employee physical fitness is an undervalued competitive asset.
-- Corporate physical fitness programs can decrease absenteeism by 30%.
According to the report, an in-house gym will encourage employees and help them spend more time at the office. (Google, for example, has in-house gyms.) 
I've attached an estimate from a building contractor that we've used in the past.  Since employees can use the nearby restrooms to change, the cost is considerably less than hiring a new employee. 
Shall I go ahead and submit the contractor's invoice?  If not, when can we discuss the idea further? 
What this gets right:
 Subject is relevant to the decision-maker and appropriate to the sender's job role.
 States the problem immediately and leads the decision-maker to "feel the pain" that the idea will cure.
 Communicates quickly what's being proposed.
 Shows that sender has done his homework and provides a proof point for the idea's validity.
 Sender has done the groundwork and minimized risk by going with a known contractor.
 Anticipates and answers two likely objections (the lack of changing facilities and the overall cost.)
 Moves to close the deal. Note that all that the decision-maker need do at this point is say "OK."
 Lays the groundwork for more selling, just in case the decision-maker isn't yet convinced.
The above article was adapted from instructional materials supplied by Better Communications, a company that helps businesspeople communicate more effectively with the written word.