Here, at a minimum, is what you should look for in a tax preparer:

1. Look for a tax preparer who is an EA (Enrolled Agent) or CPA, since tax preparation under either designation is regulated by the IRS. Ask the tax preparer if he or she practices under IRS Circular 230 (which sets out the ethical and professional requirements for tax preparation). If the preparer does not know what you are asking about, go elsewhere.
2. Your tax preparer should be available year-round, and able not just to prepare your return, but help you with any IRS questions or investigations regarding the return. Your tax preparer should be qualified and experienced in representing clients before the IRS.
3. Beware of promises of a "larger refund" especially when made without even considering your particular facts and circumstances. A good tax preparer should always be on the look out for ways to legitimately lower your tax liability, but claiming he can do so for all people in all situations is just plain hype, if not outright unethical. Even worse, if the "bigger refund" guy also offers refund anticipation loans, run -- don't walk -- away.
4. If a tax preparer sets the fee as a percentage of your refund, go elsewhere. This is known as a contingency fee and is a violation of the tax prep standards set by the IRS for EAs and CPAs.
5. If the tax preparer refuses to sign the return, without informing you of the reason, this, too, is unethical. EAs and CPAs can refuse to sign a return, if they have reason to believe that the information provided is incorrect or fraudulent, but they also must inform the taxpayer the risks and consequences of filing an incorrect or fraudulent return. What you need to watch for, is a preparer handing you a filled out return, without signing it for the simple reason that he or she does not want to deal with any followup or be held responsible for any inaccuracies.