Fortunately using greylisting with Postfix is rather trivial. You would need to install the postgrey package first:

$> aptitude install postgrey

Right after its installation the postgrey daemon will start running in the background and listen for requests on TCP port 60000. To make Postfix use that policy daemon you have to add it to the smtpd_client_restrictions setting in your /etc/postfix/main.cf. Example:

smtpd_client_restrictions =
    permit_mynetworks
    permit_sasl_authenticated
    reject_unauth_pipelining
    reject_rbl_client bl.spamcop.net
    reject_rbl_client dynablock.njabl.org
    reject_rbl_client zen.spamhaus.org
    reject_rbl_client list.dsbl.org
    check_policy_service inet:127.0.0.1:60000