Let's say you've the platform property which duplicates in each project file:
<Platform>x86</Platform>
You can extract this property from ALL project file in a single CommonProperties.properties file:
<?xml version="1.0" encoding="utf-8" ?>
<Project
ToolsVersion="4.0"
xmlns="http://schemas.microsoft.com/developer/msbuild/2003"
DefaultTargets="Default">
<PropertyGroup>
<Platform>x86</Platform>
</PropertyGroup>
</Project>
And then just import it in ALL project files:
<Import Project="CommonProperties.properties" />
EDIT: Multiple platform support
<Platform Condition="'$(Platform)' == 'Win32'">x86</Platform>
<Platform Condition="'$(Platform)' == 'x64'">x64</Platform>
Useful links: