There are two ways:
2. In SP 2010, we can create a custom action feature. If you are aware of developing features then use this technique and it will have advantage of not customizing the master page.
While developing feature, in Elements.xml file use appropriate custom action XML as mentioned below. Use location attribute with value "ScriptLink".
<CustomAction Location="ScriptLink" ScriptSrc="<specify path relative to _layouts. Do not include _layouts in path>" Sequence="<Provide proper sequnece number>" />
You can find more information here.