Define global template helpers in underscore. Example:

_.addTemplateHelpers( {
	iff : function( condition, outputString ) {
		return condition ? outputString : "";
	}
} );

Now in an underscore template:

<script type="text/template">
	<div class="button <%= iff( isHightlighted, "highlighted" ) %>">
		My button
	</div>
</script>

Using with Backbone.Subviews

This mixin comes in handy when used with the Backbone.Subviews View mixin to insert subviews into a view template:

_.addTemplateHelpers( {
	subview : function( subviewName ) {
		return "<div data-subview='" + subviewName + "'></div>"
	}
} );

Now to create a subview from your template, you just need:

<script type='text/template' id="MyItemViewTemplate">
	<h1>This is my item view template</h1>

	<%= subview( "mySubview" ) %>
</script>