Thou shalt be polite—It should go without saying but often doesn't. Don't demand; request. Be sure to address the person politely. E-mail them using the contact information they request. Be sure to use their title if they're a professor, doctor, etc. And if they say on their Web site that they can't answer e-mailed questions, don't ask! Sometimes folks are just really busy.

Thou shalt make it clear what you want—You should be polite, but you don't have to be obscure. If you're looking for a particular fact, say so: “I'm trying to find out who kicked the longest field goal in NCAA football history.” If you're trying to get more information on a particular point, describe it clearly: “I'm trying to understand how subsonic communications between elephants were discovered in the first place. Since we can't hear them, how did we become aware they existed?” Make sure the expert knows what you're asking for.

Thou shalt give a reasonable amount of time for an answer—Part of courtesy is giving the expert plenty of time to answer your question. Try to give them three or four days. Sometimes it might take longer. Don't e-mail them and say you have to have the answer in an hour (well, feel free to say that, but don't expect a response). On the other hand, if you have a deadline say so, so the expert knows you need to hear back from them within a certain time frame.

Thou shalt not ask the expert to do thy homework—If you write to an expert on Shakespeare with a request like, “Please send me 500 words on the underlying themes in Romeo and Juliet,” don't be surprised if you get no response. It's one thing to ask for help; it's another thing to ask someone to do your work for you. I've found in my research that an expert is a lot more willing to help if you describe what you've done already, where you've tried to get the answer, and why you're contacting them.

Thou shalt not get in a twist if the expert does not answer—Sometimes people get busy, or tired, or their computer breaks, or they can't get their e-mail, or they get carpal tunnel syndrome. Sometimes you won't get an answer. Don't take it personally! People have lives and those lives don't (usually) revolve around making sure their e-mail gets answered. If you want to, send one follow-up e-mail—sometimes legitimate e-mail gets caught in a spam filter or otherwise undelivered—but then leave it alone.