Some common mistakes to avoid when billing clients include: Failing to clearly communicate billing terms upfront. Not keeping accurate and up-to-date records of billable hours. Not billing for all the services provided. Submitting bills late or infrequently. Ignoring or delaying follow-up on overdue payments. It's important to remember that billing mistakes can damage your reputation and financial stability, so it's crucial to stay organized, transparent, and proactive in your billing process.