If someone's profile has caught your eye and you are sending a first email, the goal is to make him or her feel valued. Don't send generic emails that just say "nice profile, let's chat" or, even worse, "can we hook up." Demonstrate that you have read the person's profile, and include comments or questions that show that you are paying attention to what he or she wrote, such as "I love rollerblading, too" or "I'm so impressed that you did Peace Corps -- I'd love to hear more about that."
If someone took the time to write to you, it is good manners to write a response, even if it is just to say "thanks for your flattering email, but I don't think this would be a good match." If you are, in fact, interested in learning more about this person, respond in a way that shows you have carefully read his or her profile and email.
If you write to someone and he or she doesn't respond, try not to take it personally; many dating sites charge members for the privilege of reading or responding to their email, and this person may not be interested in paying for the service right now.
The online dating experts at Match.com say that "honesty is your best relationship tool," and recommend using current photos of yourself and a truthful description in your profile. That goes for emails, too. Although it's smart to present yourself in a good light, don't pretend to be taller, thinner, richer, younger, or better-looking than you really are. Remember your goal of finding a real relationship---and that the person you are emailing wants the same thing, not a fanciful web of lies.
You aren't under any obligation to divulge your real name, address, phone number or any other personal information until you are sure the other person is a reasonable human being and not, say, a potential stalker. Don't push people to give out their phone numbers, especially if you haven't already exchanged a few emails.
Once you have exchanged a few emails with someone, don't just "disappear." If you aren't ready to take the next step of a phone call, say that. If you aren't interested in pursuing a relationship with this person anymore, then write a gracious goodbye. If you have started dating someone else you've met offline, explain what happened and wish the other person well.