If you are offered a position you feel you "should" accept because you fear you will not have other options, think carefully before you accept. Do not accept an offer before you are ready. If you are being pressured to accept and the employer is firm about needing to know before you're ready, seriously consider listening to your hesitancy and turning the offer down. Acceptance of an employment offer should be made in good faith and with the sincere intention to honor the commitment. Once you make a commitment to start the job, reneging on your commitment would be viewed as unprofessional and unethical.
If you’ve given yourself enough time to truly evaluate an offer and determine the position is not a good fit for you, demonstrate your appreciation and respect for the organization that made you the offer by turning it down as quickly as you can. Call the employer to verbally decline the position or write a brief, concise letter releasing the position. In either case, remember that this truly is a small world and you might meet these people again.
Sample Rejection Letter (PDF)