There is no better way to prove ROI than to set clear objectives, outline them in terms of goals and expectations, and delineate methods to reach those objectives. Just like your creative approach, this will differ for every company and in every area of business. Some businesses will have a more difficult time in tying specific objectives to methods and processes. Don't let this discourage you.
Some businesses lend themselves to an objective-based ROI approach; they have a shorter sales cycle, a smaller supply chain, and are easy to quantify in terms of tracking units sold. But some processes will be very difficult to "pin down" in terms of tracking numbers. They will require a different approach and more time in determining and developing areas to be quantified and measured.
Cross-disciplinary brainstorming can be helpful in clarifying objectives. Set aside a time to meet, and pull in team members from marketing, sales, customer service, accounting, and web development. You may also wish to bring your agency and/or in-house advertising people on board so that they can contribute to the process.
Make sure everyone is aware that you'll be working together to better define and measure ROI. Getting a good return on marketing investment is not "just a marketing thing." Everyone needs to be part of the process. Each subsequent time you meet to discuss ROI, close the meeting by ensuring that you are all on the same page with regard to expectations.
