Most users repeat their current job title. Instead, use the opportunity to differentiate yourself from your competitors. You have 120 characters. Tell your story and make it compelling. Succinctly say what you do, who you do it for and how you can help and why they need to consider you.
Here are a few good examples:
“I work with business owners to improve financial reporting, reduce tax and plan for succession”
“I am a CA who enjoys helping small to medium sized businesses solve accounting, tax, and other business problems”
“I develop growth strategies for Vancouver business owners”
“Assisting professionals leverage LinkedIn”
This is your opportunity to capture the readers attention. Use keywords they can relate to and may use when searching for a professional with your experience.
One last thing. Change your Headline periodically. Reword it using different keywords. This will maximize your exposure enabling you to appear in different search results. And, it keeps it interesting.