http://www.erowid.org/chemicals/dxm/faq/dxm_faq.shtml
Make sure any DXM preparations you try again DO NOT contain pseudophedrine, acetamenophine, or basically any other OTC medications for the common cold. Antihistimanes are a danger as well. So stay away from any medications that say cough AND COLD / non-drowsy. Best bet is: Robitussin Maximum Strength Cough
linky
I recommend at least 8 fl. oz. If you have a hard time stomaching that much, Robitussin does sell just DXM in gelcaps at most pharmacies, best bet if you don't like the taste of cough syrup and don't mind swallowing 12-20 horse pills. Good luck, be safe and have fun.