1. The client is paying you to do a job.  Do it and do it well.
2. Only take on projects you know that you can complete in the given time with the given budget.
3. Communicate with your client.  Do not hide anything from them.  If you have questions, ask them.  Do NOT assume anything.  Not getting answers in email? Pick up the phone and call.
4. The client doesn't care about the books you've written, only that you can produce results - so do it (see #1).

My primary client recently had some bad luck with a contractor.  The guy said one thing, but did something entirely different.  He routinely billed big hours, but only produced a fraction of what was agreed to in the time given.  This guys incompetence put us behind, which is really not cool.

This guy had a 7-page resume, has authored several books and spoken at conferences.  I should submit this to the Daily WTF. :-\

<ICCA_Plug>
As a member of the ICCA, I follow the ethics and professional standards of that organization.  Maybe this is why I'm the only contractor they've brought on this year that has not screwed them. :-)
</ICCA_Plug>