Good Morning,
As you probably know drinking plenty of water will help to thin the phlegm so that it moves more easily. As long as you are not on a fluid restriction you should be drinking 6 to 8 8-ounce glasses of non-caffeine non-alcoholic fluid daily. Clearing your throat can irritate your throat and make it sore. When you feel the need to clear your throat sip some water to clear the phlegm.
An antihistamine can dry up the postnasal drip. However, it may make the phlegm thicker and more difficult to move. A decongestant like Sudafed (pseudoephedrine) can thin the postnasal drip and decrease the swelling so it drains easier. A combination antihistamine-decongestant will do both. Check with your doctor to see which would help you more.
Good luck,
Kitt