首先应该恭喜你有更好的选择!:)

When you have decided to reject a job offer, you may want to let the employer know in writing that you are declining the offer. Your letter should be polite, brief, and to the point. You don't want to burn bridges and this employer may have a better offer for you down the road. So, don't get into any specifics. Even if the hours are awful, the work environment is terrible or the pay isn't enough to make ends meet, don't mention it.

You should include the following:

• Thanks and appreciation for the offer
• Written rejection of the job offer

Address the letter to the person who offered you the position. Include your contact information and phone number, even though it is on file with the employer.

Make sure that your letter is well written and does not contain typos or grammatical errors. Even though you are declining the job, you want to make sure all your correspondence is professional.

Sample:

Date

Dear Mr. Gilhooley,

Thank you very much for offering me the position of Marketing Manager with Hatfield Industries. It was a difficult decision to make, but, I have accepted a position with another company.

I sincerely appreciate you taking the time to interview me and to share information on the opportunity and your company.

Again, thank you for your consideration. 
Signature 
Susan Markstone