What to do once you've made a decision
Once you have decided which job offer to accept, you should send a letter to the chosen employer accepting the offer. The letter should detail the specifics of the offer they made to you so there are no questions. Include your understanding of position, starting date, salary and benefits, and any other arrangements the employer told you when making the offer. Here's a sample acceptance letter for your reference.
Just as important as the acceptance letter is the refusal letter. You don't want to leave an employer waiting for an answer once you know you've secured a different job that was just a better fit for you at this time. You never know - you may want to apply for another job there one day and you don't want to burn any bridges. Need ideas? Look at this sample refusal letter.
What NOT to do
Above all else, do NOT accept an offer if you know you're going to continue your job search. Backing out of an accepted offer is not only considered bad form, it can come back to haunt you if you hope to work for that company in the future.