Delivering What You Promise
In the publishing business, your word is your bond. You're expected to tell the truth about your published credits and any endorsements you might have received; you're expected to deliver original work, with nothing plagiarized or otherwise lifted from another work without the appropriate permissions; you're expected to deliver your work in the time you agree to. Do all these things, and agents and editors will regard you as a pleasure to work with. Keeping your promises is a key element of building a professional image.
Being Honest
You've probably heard stories about college “professors” who turned out to have fake credentials, airline “pilots” who obtained a uniform and managed to take off in a Boeing 747 with no previous training, and “doctors” who performed surgery without ever setting foot on a medical school campus. Likewise, there have been authors who flesh out their bios with false credits or who claim to be experts in a field when they really have no expertise. Often, these pretenders justify their actions by saying it was the only way they could get an agent or editor to take their work seriously. But in the end, the damage to a dishonest writer's career far outweighs any gain they might have received thanks to these tactics.
Agents and editors appreciate honesty in writers as much as any other quality because it saves them enormous difficulties down the road. An agent can always talk up a client's talent and potential, even if the client hasn't yet been published. An editor can make allowances for an unpublished author who doesn't know what to expect out of the publishing process. But neither will be inclined to help or work with someone who has misled them.
Being honest extends to your own concerns and worries, too. If a deadline in a proposed contract seems too tight, tell your agent or editor why you think you'll need more time. If, in the middle of a project, you run into a problem that will affect your ability to complete the work on time, tell your agent or editor right away. If you're stuck on a chapter or scene, let your agent or editor know; they might (and usually do) have some useful advice that will free you and keep your book on track.
Agents and editors have heard every imaginable excuse from writers who just can't seem to meet their deadlines. But they also know that life can interfere with the best-laid plans. The earlier they know about problems, the better; early warning gives both of you time to work out a solution.
Being Responsive
Just as writers get frustrated and sometimes cranky about going weeks or even months without hearing from an agent or editor, agents and editors get justifiably edgy when their authors suddenly go incommunicado for indefinite periods. Consistent failure to reply to e-mails or return phone calls causes needless anxiety. You always should respond as quickly as possible, even if it's just to say that you're swamped right now and will get back to the agent or editor as soon as you can. Of course, you have to do that second follow-up; that's part of keeping your promises.
If you're going on vacation or a business trip, let your agent or editor know that you'll be unavailable during that time. It can be as simple as sending an e-mail to that effect, or using the “auto-reply” function if your e-mail program offers it. If you can't let your agent or editor know about your absence beforehand — if, for example, you have to fly to your parents' hometown because your father suffered a heart attack — make sure you notify your agent or editor once the crisis is past.
If you just can't get in touch with your agent or editor, and you come back to find e-mail or phone messages, explain why you didn't respond earlier. Agents and editors have vivid imaginations, just like the rest of us, and unexplained silence from an author can conjure up all sorts of awful scenarios. A simple explanation, even after the fact, is an antidote to those imaginings.

