Make an appointment with your manager to resign, and don’t tell the reason for the meeting when you make the appointment. Hand them a written letter of resignation (see sample below). Tell them that you have reached a final decision and that you would like their respect and support of that decision. Tell your manager, verbally and in writing, that you have given this job move a great amount of thought and will not "entertain" a counter offer. Be firm about your decision to leave and let them know you will work hard for your last few weeks.
Please do not mention that you were working with Global Search because this is a small industry and we have many contacts.
It is with mixed emotion yet firm conviction that I write this letter of resignation from COMPANY. My association over the last few years with this firm has been a significant part of my professional and personal life. Please understand that I have made my decision after considerable deliberation, only because an outstanding opportunity presented itself to considerably further my career and help me achieve my own goals. I am therefore resigning from COMPANY effective DATE. Know that it is my intention to work diligently to wrap up as much as possible in the next few weeks to make my resignation as smooth as possible. I hope that you and your peers will understand and accept my decision, and that I will have your support in making this change as easy as possible for the staff and department.
Sincerely, (Type and Sign your full name here)