If you want to have a Free and Paid version of your app in the same project without using a 'Trial' version, this is how I do it:
Each project is assigned a single ProductID which distinguishes the app from other apps at install time. You could create a second project and link to all the files in the first project, but that would require maintenance as the project grows. My solution allows using the Build Configuration to select the free or paid app to build.
First you need a separate ProductID for each version of the app. This ProductID is declared in the manifest file 'Properties/WMAAppManifest.xml'. So the first step is to create two versions of WMAAppManifest.xml. I call them WMAAppManifestPaid.xml and WMAAppManifestFree.xml.
In each of these manifest files, provide a separate GUID for the ProductID and also change the Title of the free version so you can tell them apart when they are installed.
Next we need to add two new Build Configurations in the project. I call them ReleaseFree and DebugFree.
Next you add a few Pre-Build Events to all the build configuations to copy the appropriate manifest file:
if $(ConfigurationName)==Release copy $(ProjectDir)\Properties\WMAppManifestPaid.xml $(ProjectDir)\Properties\WMAppManifest.xml
if $(ConfigurationName)==Debug copy $(ProjectDir)\Properties\WMAppManifestPaid.xml $(ProjectDir)\Properties\WMAppManifest.xml
if $(ConfigurationName)==ReleaseFree copy $(ProjectDir)\Properties\WMAppManifestFree.xml $(ProjectDir)\Properties\WMAppManifest.xml
if $(ConfigurationName)==DebugFree copy $(ProjectDir)\Properties\WMAppManifestFree.xml $(ProjectDir)\Properties\WMAppManifest.xml
You should now be able to build either the free or paid versions of the app by simply changing the Build Configuration.
Next, to allow for actually making the free version different than the paid version, such as limiting features, showing different pages etc., you need to add a Conditional Compilation Symbol, such as FREE_VERSION to the two free build configurations.
then you can simply use compiler directives to change the code such as:
#if FREE_VERSION
s = "My App Free";
#else
s = "My App Paid";
#endif