Hello and welcome.
To begin with if it is thick you're not drinking enough water.
I have been drinking mullein tea, that has reduced my phlegm by 50% or better.

If you're talking about trying to clear mucus but having troubles coughing it up. Or to help clear a mucus plug. My doctor gave me a Aerobika to use. I breathe through it 4 times and it loosens it up and I cough it right up. Sometimes I have to do 8 breaths to get it all out. Along with a huffing cough.

They sell other brands on line that have the ball bearing in them to help vibrate and loosen the phlegm. If it is severe you may need a percussion vest. Talk with your pulmonologist he/she can best advise you.

I love coughing up mucus. Reduces pain and pressure in my lungs and definitely helps me breathe easier.
I drink 16oz of water with unflavored electrolytes twice a day along with all the other water throughout the day. I also monitor how often I urinate and the color to ensure I am properly hydrated.
It is important to know that you can "wash" vitamins and minerals from your system.
Your doctor is the best person to talk to about this.

I drink around a gallon of water a day, no juices milk soda, Just water!

Drink lots of fluids it will help by thinning the mucus so it's easier to cough up

For mucus, I take Mucinex daily. It works for me.