Keep orderly files. Auditors love to audit and lawyers love to find chinks in armor. Attention to detail is worthy of respect in these environments. Find someone who will do it for you if you can't do it yourself. Seek professional accreditation.
Accountants take an exam and become CPAs or CFPs.
Lawyers are JDs or LLBs and pass a Bar exam. Get some initials that say you are good at your profession, too. You'll never have enough time, enough money, enough staff support, enough credibility. Dress like a partner. Follow the lead of well-respected partners and make them your role models for appearance.