What are some of the most effective ways to resign from a job without burning bridges? If using a letter, do you have example verbiage?

It depends somewhat under what circumstances you are resigning. In almost all cases a letter is appropriate. If you believe you are leaving on good terms, then the letter can express your appreciation for the opportunities the company has afforded you. If the terms are not good, simply state that you are resigning your position as of a certain date. If possible give your employer at least one pay period or two weeks notice.

In some cases you might be leaving because of discrimination, harassment, or unfair labor practices. If the problem is sever, then file a complaint with the company or union along with your immediate resignation. Document everything!

Deliver the resignation in person to your supervisor if at all possible. Stand firm in you decision to resign and keep it professional.

Some resignation letter don'ts:
Don't complain about management unless you can substantiate a claim of discrimination, harassment, or unfair labor practices.
Don't complain about salary or benefits.
Don't tell whom your future employer is or when you plan to start work for them (it's not their business)

In general, give them written notice and a brief non derogatory explanation about why you are resigning, better opportunity, better fit with the job, closer to home, etc. I've been hired back by former bosses on three occasions, although only once for the same company. Most companies understand if you leave to take a better job.