Tagging the remainder of a body

I'm trying to leave a "type" tag in a body that determines the latter 
half of an XML instance.  I can't find a decent way to do this but am 
thinking I'm tackling the problem wrong from the outset.  Basically, I 
want to be able to enforce that if a type is "Event", the latter half of 
the message has an event but if the type is "Contact", the latter half 
of the message has contact information...like this:

<message>
    <type>Event</type>
    <event>
       <stuff></stuff>
    </event>
</message>

<message>
    <type>Contact</type>
    <contact-information>
       <address></address>
    </contact-information>
</message>


Am I approaching this wrong for XML?  Is there anyway to constrain the 
following nodes like this?  selector, nor choice seem to allow this 
linkage to occur...I'm not necessarily "new" to schema, but neither am I 
"advanced" in schema...so apologies if this question seems naive...

Paul Monday
Sun Microsystems