A thank you letter after you accept a job offer shows good taste, gratitude, and in general shows a new employer that they made a wise decision in hiring you.  Most new hires do not write one, so if you are one of the smart few people who do, your relationship with your new employer will get off to a great start.

When you write this type of letter, the main goal you wish to accomplish is solidifying the new relationship.  You can do this by re-stating the qualifications you have that will be beneficial to the company.  Remind the hiring manager in this way of the reasons he had for hiring you.  You are, in this way, congratulating him on his good sense, but doing so in a non-offensive, and non-egotistical manner.  Be matter of fact, and to the point, because that is businesslike and you and he are there to conduct business.  With luck you two will have a very long and mutually rewarding relationship.

If you are accepting a job in sales, marketing, promotions or a related field, then your letter should remind the hiring manager of accomplishments you've had in the past in sales or marketing.  Tell the hiring manager that you are motivated and looking forward to working with your new team, adding sales to their gross and profits to their bottom line.

If you are accepting a job in retail management, your letter should re-emphasize to the reader your strong skills in customer service, your work ethic, willingness to be an active and cohesive part of the management team, and your ability to work well with the general public and your co-workers, leading by example and presenting a positive company image to the public.

A person working in education, as a teacher or administrator, should have a letter that emphasizes a willingness to work as part of the education team, teaching your subject to students and in the process, hopefully instilling in your charges a contagious enthusiasm for learning.

Your letter may also be a place to elaborate on goals you may have during your tenure with the organization.  A salesman may mention his desire to increase sales by a certain percentage. A project manager may mention his or her desire to participate in a particular project that is important to the company's success.  A teacher may mention school organization's that he or she wishes to sponsor.

While your resume made your first impression, and you during the interview obviously made a positive second impression to have been offered the job, your thank you letter after you've got the job will reinforce those impressions, taking away any buyer's remorse or cold feet the hiring authority might have later.  It will service to convince them that they made a good decision in bringing you on board as a part of their team.