If you are going to quit, get ready.  Clear your office of embarrassing computer files, personal papers and anything else you want to keep.  As the ABA Bar Journal advises lawyers, once you resign, your employer may very likely escort you out of the office immediately with nothing more than a promise to send your belongings later.  See Bar Journal report.  The advise is for lawyers, but it applies to any white collar employee.  Many employers now escort departing employees out the front door.  

Do not solicit clients, says the Journal, before departure.  Just as it would be wise for everyone else to not solicit customers before departure.  Beware of any non-compete agreements.  

If the employer has an exit interview, do not nitpick.  Try to address systemic issues.  Try to treat your departure as a business decision, despite the many events that may have lead up to it.  

Accept offers to meet for lunch or coffee from co-workers.  Do not turn these social sessions into gripe sessions.  Whatever the circumstances of your departure, turn it into a positive event. 

Remember former colleagues’ birthdays.  Send them thank you notes for their help with your career. 

The Bar Journal does not mention this, but also, see a lawyer of you think you may have grounds for a lawsuit.  Even a lawsuit, in the end, is a business decision, too.