I use this snippet:
<action method="addItem"><type>skin_js</type><name>js/script.js</name></action>
EDIT:
Add layout update in module's config app/code/.../Your/Module/etc/config.xml
<frontend>
<layout>
<updates>
<your_module>
<file>your_module.xml</file>
</your_module>
</updates>
</layout>
</frontend>
Put it in layout update file app/design/frontend/base/default/layout/your_module.xml
<default>
<reference name="head">
<action method="addItem"><type>skin_js</type><name>js/script.js</name></action>
</reference>
</default>
I used handle default
but you can use any other handle, according to your needs, e.g. yourmodule_controller_action
.
Your js file should be in skin/frontend/base/default/js/script.js
.