Here is an example:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:mkt="http://www.marketo.com/mktows/">
<soapenv:Header>
<mkt:AuthenticationHeader>
<mktowsUserId>XXXX</mktowsUserId>
<requestSignature>XXXX</requestSignature>
<requestTimestamp>2013-04-15T00:00:00-06:00</requestTimestamp>
</mkt:AuthenticationHeader>
</soapenv:Header>
<soapenv:Body>
<mkt:paramsGetMultipleLeads>
<leadSelector xsi:type="mkt:LeadKeySelector" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<keyType>IDNUM</keyType>
<keyValues>
<stringItem>1</stringItem>
<stringItem>2</stringItem>
</keyValues>
</leadSelector>
<includeAttributes>
<!--Zero or more repetitions:-->
<stringItem>ID</stringItem>
<stringItem>Email</stringItem>
<stringItem>FirstName</stringItem>
<stringItem>LastName</stringItem>
</includeAttributes>
</mkt:paramsGetMultipleLeads>
</soapenv:Body>
</soapenv:Envelope>