Just send your POST request to yourproxy.php?url=POSTURL instead of directly to POSTURL.
I used this proxy to develop a feature that translate the whole webpage into another language using Google Ajax Language API. Sadly I found the Google’s Limit that if you POST a text longer then 5000 characters, Google Server will return a error “exceed string length limit”. I am exploring other open translation APIs, such as Microsoft Translator AJAX API. However, Microsoft’s Docs seems to be more complex ( and ugly = = ) than Google’s.