One possiblity is a low grade sinus infection that keeps draining down your throat, especially at night. If the flem is worse in the morning, then this is a good guess.
The other is an alergy to something you are eating or smelling, like a new carpet, or new paint or something new in ur diet.
You can try solving the former by making a solution of salt water and sniffing it up ur nose and letting it go in your nose and down ur throat. The salt will help clean your nose and kill bacteria. You need to do that every morning and night for about a week or 2.
If its the later, then you need to try eliminating things from your enviroment and checking for improvement. This can be a long process, and of course it might not work.
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Flem. How do I reduce it? 
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