What do you look for while searching designer for your flash ads? Here are some points which I have found very relevant:

• Experience of banner designing in Adobe Flash
• Designer’s portfolio
• Customized sizes of flash banners
• Creative approach
• Output delivery format
• Customer testimonial
• Turnaround

Do you have any other suggestions to add?

I would look for a designer who will work with you on the design, instead on one who is controlling over the design, and try's to tell you what is right or wrong.