Don’t Spam
Careful what you send out via email.  If someone hasn’t opted-in, then they’re going to be upset if you send them an email they don’t want.

Certainly, don’t send it more than once a week.  Constant emails, even if opted-in, are considered spam to your customers.  Consider RSS Communication if you need to send out frequent updates.

Be Transparent
Don’t make the mistake of pushing a product or service without being completely upfront about your business dealings.  If you are the PR person for ______ be sure to state your relationship in the beginning and often.

Communicate Openly
Allow comments and trackbacks.  Listen AND respond to both positive and negative reviews.  Be willing to admit your mistakes and apologize.  Communicate your excitement at a job well done.  Be ‘real’ with your customers.  It builds those relationships that last.