We see a lot of emails here. We've written quite a few too. If you are just getting started, here are some basic tips to get started.
Make a single ask. It is almost never a good idea to ask for more than one thing at a time in a single email. Your audience is busy. If you need them to take an action, tell them. Don’t make them decide which one to take. If you have more than one ask, then you have more than one email.
Learn to not ask. You have someone’s email address. Presumably, you have it because they want to hear from you. The best way to make them regret your emails is to ask for something every time. Try to give back to your audience as much as you ask. If you’re an elected official, then tell them what’s going on in the off-years. If you’re a nonprofit, then provide some interesting news updates around your cause.
Your relationship is everything. Adding on to that last point, the best way to get people to act on your behalf is to build a good relationship with them. This cannot be overstated. It’s magnitudes more important than the title of your email or where you put the links. So treat your email as a relationship-building tool, not just a fundraising tool.
Be personable. Email is a great tool because it allows you to have a conversation with your supporters at a low cost. It allows you to communicate, deliver valuable information, and build your relationships. So write something that you’d like to read.
New Organizing Institute has a basic but decent checklist of effective email writing. It covers basic process and techniques. What I liked about it was that its simple and to-the-point and doesn't attempt to make email more complicated than it really is.