This was bugging me for a while too, the trick is to put your stackpanel within a scrollviewer.
Also, you need to ensure that you set the CanContentScroll property of the scroll viewer to True, here’s an example:
<ScrollViewer Grid.Row="1" Margin="299,12,34,54" Name="ScrollViewer1" VerticalScrollBarVisibility="Auto" HorizontalScrollBarVisibility="Auto" Height="195" CanContentScroll="True">
<StackPanel Name="StackPanel1" OverridesDefaultStyle="False" Height="193" Width="376" VerticalAlignment="Top" HorizontalAlignment="Left"></StackPanel>
</ScrollViewer>