Gone are the days of starting a career with one company and staying with it for 30 years or more. Today's workplace is more fluid then ever; according to the Bureau of Labor, 20-somethings change jobs every 18 months, while a 2004 U.S. job recovery and retention survey concluded that 75 percent of all workers are looking for a new job.

But just because everyone's doing it doesn't mean they are doing it right. If you get a new job, it's crucial that you leave your old one gracefully -- for both your colleagues and your career. You may cross paths professionally with co-workers again, and you also don't want to tarnish your reputation as an honest, upright colleague.

Here's how to make a smooth and graceful exit:

Give plenty of notice. Randall Hansen, founder of Quintessential Careers and writer of the Career Doctor column, says that standard notice is a minimum of two weeks. Consult your employee handbook in case your employer expects more warning, however.

Write a resignation letter. It's smart to draft a professional note of resignation that includes your final day of work, adds Hansen.

Don't get senioritis. Stay on top of all your assignments and duties during your final weeks on the job, says Hansen. Finish any outstanding duties and be sure to leave detailed progress reports so that the transition is easy for your boss and co-workers.

Help with the hiring process. Offer to assist in finding your replacement, and be sure to be available to train whoever that person is when he or she comes on.

Know what you're entitled to. Check with your employee handbook and benefits literature to find out what you're owed after you resign. You may be paid for unused vacation time or other benefits.

Keep your mouth shut. Bill Radin of recruiting firm Radin Associates says that "leaving a job is strictly a private matter -- waving your dirty laundry serves no purpose."

Stay in touch. Make sure your old employer and key colleagues have your contact information; get theirs, as well -- and use it. Stay in contact and maintain good professional relationships.

Say thank you. Send your boss and co-workers a thank-you note highlighting the positive aspects of your job. After all, they have played a major part in your success.