I think the best thing to do is use an example like 37signals and basecamp. They made the most simplistic 'to do' list type of app and sold it as a project management tool. Thing didn't even have due dates :] and they were wildly successful. Have them read 'rework ' by 37signals team. Their entire mantra is less, and you don't really need that feature you just think you do. They say no to pretty much every feature, it takes a lot of battling to get them to add something new.
Also, use something like pivotal tracker to track features, estimates, etc. so everyone can see what a new feature will take, what needs to be sacrificed, how it will affect timelines and you can order it's priority easily.