How can I protect my idea before I hand it over to an attorney for a patent search?

You could have the attorney sign a confidentiality agreement. However, the code of professional ethics that every attorney is required to abide by would prevent him or her from revealing the details of your invention and the information on the patent application. Any attorney who violates this code can be disbarred.

Engage the attorney first. Afterwards, his work product will belong to you and confidentiality is provided by attorney ethics/licensing rules.

If you are concerned about your patent attorney stealing your idea, you should not hire that patent attorney. Patent attorneys are in the business of protecting your intellectual property not stealing it. Any attorney that is disreputable in that regard will not be practicing law for very long. Take you time and find the right patent attorney that will work with you and your budget.

You really can't. However - attorneys are bound by ethical rules that require them to hold your confidential information forever. Fear not - go to your attorney, order the search, and be off to patenting your invention.