Use the "submit" method of the Form object. You should always give your form an ID:
<form id="myForm" action="myServerSideCode.php" method="post"></form>
Then, you can reference and submit the form with a script that looks like:
It is possible to do much more than this. You can build your entire http send or post response, with message and header fields. And you can do it reasonably quickly using html DOM traversal by element type (which you can call on any html element or the html body element) and you can process the content of the elements using browser methods to make them URI compatible.
This approach is very useful if you have a specific reason for it, but it is much more complicated than the technique described by Thomas.