Make sure you give plenty of notice so that agencies can research and develop proposals outlining the best possible ideas for you. Be available to answer questions while agencies prepare their responses.
Make it clear what you expect to see in a presentation – do you want detail on all the elements of their proposal, with delivery plans, or do you want some big picture ideas to discuss with them, to get more of a feel for their creative approach?
When you meet agencies make sure they are all given an equal chance – so pay attention to how much time they are given and who is on your panel.
Schedule in time for questions and discussion with each agency you meet.
Look for ideas, ability and a team you can work with, rather than seeing proposals as a definitive project plan. A pitch is a presentation of what an agency might do for you, not a rigid manifesto. If an agency has the experience, flexibility and temperament to work with you effectively, plans can be finalised between you.
Be prepared to feed back to agencies after the process. They are likely to have invested a good deal of time in developing their pitch and will appreciate knowing how you reached your final decision.