I can't seem to cough up the mucus, and what OTC medicine can help the best?

Careful... You don't always want to suppress a cough, especially if there is infection present. So, before taking an otc cough med, it is important to know the etiology of the cough to determine the best med to take for that particular cough. Cough etiologies include acute bronchitis, bronchospasm, gerd, post-nasal drip and sometimes a combination of factors. All these have different treatments. See your doc.