Acidic food and juices may not be good, including citrus (except maybe lemonade depending on how you do with it), tomato-based, and spices. It is not just reducing phlegm but getting rid of it, so you want to stay hydrated and not let it dry up. Pineapple and papaya juices, warm drinks, lemonade, coffee/tea are all worth trying, in addition to medications as Lisa mentions.
It can help to proactively spit into a paper towel before things get bad, as long as you keep up with hydration whether by mouth or tube.
You want to have clean furnace and BiPAP filters/hoses and adjust the temperature/humidity of your space to what's not irritating, part of how phlegm builds up.