The more information you stick into one email, the more likely your recipient is going to lose focus on what's important. Here are a few ways to keep your email relevant to the task at hand.
Use Descriptive Subject Lines
We already talked about the importance of your email's subject line, and how useful it is for keeping concise messages. Even if you're not using the End of Message (EOM) trick, though, you want to keep your subject lines descriptive. Email subjects like "Question for you" or "Important!" are pretty much useless to the recipient.
Instead, use a subject that clearly defines what the email is going to be about. As Gina mentions in the Lifehacker book, you can still use certain descriptors as tags (i.e. "Important!: H.R. needs your new contact information"), but you want to make it easier for your recipient to differentiate between it and other "important" emails.
Create New Threads when Changing the Subject
Similarly, if you're having a conversation with someone over email, it can be easy to go on tangents. When you do, you want to start new thread rather than continue the old one. Blogger Philip Guo puts it best:
If your reply is not relevant at all to the subject line, start a new thread with a fresh subject line which more accurately reflects the email's actual contents. It's annoying to open an email thinking that it's about "Re: Deadline reminder" when it's actually really about "By the way, how do I log into our servers?"
Not only does it help the person you're communicating with organise their thoughts, but if you ever have to find that conversation later on, it'll be easier to find if the subject line is always relevant to the message's content. Just like you want to separate actionable items, you want to keep your conversations separate as well.
Keep it Concise
We've already talked about the importance of concision, but the easiest way to keep your messages relevant is to keep them brief. The shorter your message, the fewer "hey, how are you doing" and other off-topic rants you're likely to throw in unnecessarily. Summarize the important points, give them something to act on, and be done with it. You'll be a lot more likely to get the desired response if you don't stray from your message's purpose.