If it's non-productive, don't take Sudafed (or other drying agent). Take a cough syrup with both guifanesen (to loosen up the flegm and make it easier to cough, so you don't hurt yourself trying so hard) and detromethorphan (to help suppress unnecessary coughing).

Also, dark chocolate has been found as effective as cough syrup in controlling coughing. So eat some dark chocolate.