Go to any appellate CLE and you'll hear lots on argument, answering the court's questions, briefing, and the like. But you're not likely to hear anything about dressing for the date. Below are a few quotes from appellate judges and lawyers about the ethos of appellate dress.
Try to dress well for oral argument. Make the judges think you are a serious person who takes the court seriously. Hon. Richard A. Posner, Convincing a Federal Court of Appeals, 25 Litig. 3, 62 (Winter 1999).
Don't get slack about your appearance at court. Every day you are on show. So show them. Make sure the clothes you wear make you look fantastic. Looking fantastic makes you feel like a winner. Tribunals can't help themselves -- they take people who look like winners very seriously. And you have not opened your mouth yet. Elementary human psychology. Iain Morley, The Devil's Advocate: A Short Polemic on How to Be Seriously Good in Court 26 (2005).
The Hon. Barbara Gamer, one of San Diego's Superior Court judges, once quipped that in her court "the attorney who wear polyester has the burden of proof." Keith Evans, The Language of Advocacy 56 (1998).
The skilled attorney will always be neatly dressed, will sit properly at counsel table, and will rise resolutely when addressing the court or the jury. The novice will often be slumped in his or her chair the entire time that lawyer is either questioning a witness or argument a point to a judge. Judges notice this. Roberto Aron et al., Trial Communication Skills § 28.18, at 28-22 (1996).