David Silverman writes a great blog called "Words at Work" where he discusses communication in professional settings. His latest posting, Did your Email Get Lost In Translation? covers how to make sure your emails are clear, concise, and provide meaning to your reader. Another recent posting titled How to Ask a (Near) Stranger for Help, addresses how to ask for help in a respectful manner that helps convey how this benefits all parties. If in doubt, be deferential.
These pieces of advice are important in not just employer settings but also any professional setting -- communicating with professors, law school administrators (such as Law Career Services or Student Affairs, to name a couple), networking contacts, alumni, and professional organizations. If in doubt, treat communication opportunities as professional, and err on being clear, concise, and conservative.