I have a renovation project that I have entered the building in a existing phase. I used the demolition tool to show work removed on a demo plan. I created a new construction plan and set the phase to "New Construction and the phase filter to "Show previous + new" . The problem is that all work to be demolished appears as existing and new work appears on the demo plan as existing.
You may want to take a look at this article in the wikihelp which ex-plains the basic phasing workflow. it also has a sample project attached to illustrate how to set up phasing in a project. The articel also includes a video explaining phasing.
http://wikihelp.autodesk.com/Revit/enu/Community/Examples/Example_Project_to_Illustrate_Phasing