Bigger isn't Better: Don't contact the lead partner of a large firm or the CEO of a material manufacturer for an inquiry that is likely to be handled by someone with a different role. Chances are, they're busy doing different work, and may overlook your request.
Find the Right Contact: Spend some time on the firm or company's website. If there is a staff listing, try to determine who might be your "best bet" for correspondence. In an architecture firm, this might be a records manager, archivist, or administrative professional. When making material requests, a distributor may be your most likely source of samples.
Or, Go Generic: If you aren't sure who to contact directly, use the general contact info (an info@ email address, or a contact form, for example). If you do use a form, it may be a good idea to keep a record of your request date, so that you can follow up if you don't get a response.
Use a Phone: The fastest way to get what you need may be to speak directly to someone at the firm/company. If you are uncomfortable calling, consider making a small script to prepare to introduce yourself and your information request. Be prepared to answer questions about how you'll be using what you request, as there may be different policies for what can be shared for publication/web use vs. private/class research.
Subject Line: The subject line should concisely convey your purpose for writing. Try to consider the recipient's point of view. A subject line that states "Request for Site Plans" is more informative to the recipient than a subject line that simply includes the site name.
Greeting: Even if you are writing a very short email, include a greeting. If you know the name of the person, include it. Avoid casual greetings like "Hey."
Length: Keep your email as concise as possible, but do include essential information. A request for information should be specific ("I am writing to request plans and sections for Building X), but should also include the purpose of your request (for publication? for use in a class assignment? for inclusion in a web site/blog?)
Font and Images: Avoid ornate, playful, or colored fonts; these simply distract the recipient from your actual message. Avoid overusing bold and italics as well, which make an email look cluttered. If you include images, ensure that they are not excessively large files.
Emoticons/Emoji: Do not include emoticons in a professional email; save these for personal correspondence.
Spelling and Grammar: Just because you are writing an email does not mean you should be sloppy about spelling and grammar. Edit your email carefully before sending it. An error-free message tells the recipient that your email should be taken seriously.
Closing: Sign off with a brief "Thank you," "Best," or other simple send-off, and then your name. Most email accounts let you embed a signature with your name, title, and contact information into every email. This is a terrific way to make each correspondence more professional.