Had flu-like symptoms (tiredness, mild ache) 2-3 days ago; that subsided but still intermittent cough w/thick yellow-brown phlegm. What should I do?

Sounds viral. Drink plenty of fluids and get eight hours of sleep every night! Eat healthy and take some decongestant/ expectorant if allergy medicine does not help with coughing from Postnasal drip. See doctor if condition worsened, you may have a bacterial infection that requires antibiotics.