Which target property you want to animate is the question you have to ask yourself ! Do you want to change the rectangle's Width ? Its Opacity ? ...
Best practice... I don't know, but that's one way to do it :
<ControlTemplate TargetType="Button">
[...]
<ControlTemplate.Triggers>
<EventTrigger RoutedEvent="Button.Click">
<EventTrigger.Actions>
<BeginStoryboard>
<Storyboard TargetName="test" TargetProperty="Width">
<DoubleAnimation From="0.0" To="10.0" Duration="0:0:2" />
</Storyboard>
</BeginStoryboard>
</EventTrigger.Actions>
</EventTrigger>
</ControlTemplate.Triggers>