After going through that phase I backed off and don't really try to push myself to read fast - but those efforts helped set a different pace and I still read faster than most people around me.
Some things that I think are important: (just one guy's opinion)
1. Try to be broadly curious - an interested brain remembers better.
2. Develop your vocabulary. Particularly if you are reading something outside of your field. Textbooks often have a vocabulary section in the back of the book. Take some time to read through that before tackling the book.
3. Comprehend the structure before diving into reading. It is useful to read through the Table of Contents before starting to read a non-fiction book. Having a sense of the structure in advance helps you with memory and context for specific facts.
4. Read every day.
5. Read lots of different kinds of materials.
6. Allocate your time and attention. Give yourself permission to speed up and even just scan when it feels appropriate but also permission to slow down and focus on comprehension and integration with what you already know when you get to parts that challenge you or feel particularly important.