There are three ways to do this:
1. Use SSL. Then just pass the sensitive parameters in the form and trust the encryption to keep them secure. Doesn't work with GET requests because the sensitive parameters show in the URL bar. If the initial posting of the values needs to be secure, then you need SSL anyway. This doesn't work if the sensitive parameters need to be concealed from the user because they could view source.
2. Use encryption to protect the parameters. Crypt::CBC is the generic interface. The server encrypts the data with its private key, passes the ciphertext as a cookie or form parameter, and then decodes it with the private key.
3. Store the values in the database or filesystem. Pass around the unique identifier to find the data in the database.