How to get rid of crackly voice?

I personally love to sing and I have a high tenor voice. But due to my allergies and post-nasal drip syndrome, my voice tends to get very crackly and buzzy from phlegm buildup in my throat. I take allergy medication and drink plenty of water when I sing but I still find myself constantly clearing my voice in between pages. Is there anything else that I can do to help with this problem?