First, you can use the XML-Layout-Update of the respective page (or other layout updates in every layout xml file ):

<reference name="content oder right oder left">
    <block type="cms/block" name="blockname">
         <action method="setBlockId"><id>BLOCK-ID</id></action>
    </block>
</reference>

Or you can use the following placeholder:

{{block type="cms/block" block_id="BLOCK-ID" template="cms/content.phtml"}}