Use operational measures. It's admittedly difficult to draw a straight line between most investments and the financial impact. Fortunately, every business has operational measures such as customer service, cycle time and product quality that, over time, will result in financial impact. These measures can be identified by examining the underlying drivers of the financial results. IT improvements that increase speed of service are easier to commit to and measure than the resulting increase in sales.
Effective leaders ask their organizations to deliver what is believed to be impossible.
When I was a practicing CFO, I once asked my controller to prove that his request for a new financial system could pay back within a couple of years. I thought I had killed the request. Imagine my surprise when he came back with a justification based on tax savings created by the system's more sophisticated fixed asset processing. Even boring replacement projects can be made exciting if the team is challenged appropriately. Stage the funding.