1. Only ask one question- If there are five questions in your email, imagine how much time it will take for a reader to type out a response to all of those different questions. The reader is going to get overwhelmed. You'll get responses more often with one question.
2. Keep it short- I know you want to explain every detail of why you are emailing someone, but if they open an email and it's two pages, the odds are pretty good they won't read most of it, let alone reply.
3. Make it relevant- Explain as succinctly as possible why what you're talking about is important or relevant to that person.
4. Clear email subject- 35% of email recipients open email based on the subject line alone. During my internship this summer, my boss called me out on this. I sent him an email with the subject of just "question". He explained that he can more effectively respond to emails when it has a clear subject line. For example, if the email subject makes it clear that the email is about a meeting we are having an in hour, it lets him know I need a fast response with out having to read the body of the email.
If before you start typing away at that next email you ask yourself, "how do I make this email easy to respond to?", you're bound to get more responses to your emails.