As in, asking the ARB for permission to construct a non-standard architectural element? Find out what is important to the board and address it. (Just like a client presentation)

Wear black.

Practice your speech carefully and be brief. Is there a time limit to your presentation? If so, time your speech. Make sure you have enough time to hit your main points. Be deferential. Anticipate any and all questions. Have supporting images to address any questions you could anticipate.

Do you know the ARB members? How they lean? Who might potentially support your project? Meet with the person most likely to you in anticipation of the meeting to familiarize them with the project. They may advocate on your behalf if they become invested in it. Know what you could compromise on and what you cannot and why. If needed, toss them some compromises that are non-essential to you.