How do I post query parameters in the request body?

I'm trying to put parameters in the body of a rest post request and I'm not having any luck. The parameters need to contain xml documents which can be large. If I leave the parameters in the path of the request, the request fails because the length of the uri exceeds what the server can support. I've followed the steps shown in the "Understanding REST parameters" section in the SoapUI help and I don't get the same results as what's shown. If I check "Post QueryString", the parameters don't appear in the request body (shown on the "Raw" tab) and the request fails with an error that the service did not find any parameters. Am I missing something? Please let me know if there's a detail that I'm missing or something else.

Thanks in advance - Charlie