Use the secure server to collect sensitive information, e.g. credit card details.

This is how Worldpay operates. Your form submits fields to a script on there server which, after processing the cc details, calls a callback script on your server with all the fields on the original form passed back.

If the secure server doesn't always need to be called/used this could be handled by jscript in the original form.

There are other ways to do this but the first point above is, I believe, the key.