There isn't actually any kind of 'trick' to it. Read a lot, attempt to read more quickly, and to skip over inessential or repetitious material. Sometimes it can help you get the hang of the process if you attempt to read a very familiar book again, and push yourself to do it as fast as possible.

Mostly it's just a matter of habit. People read slowly not because they can't read quickly but because they're accustomed to their own, normal pace. Once you get in the habit of reading quickly you'll have to make a deliberate effort to slow yourself down to enjoy something you're reading just for pleasure.

When I was a kid I developed a bad habit of burning through everything I read, even the fun stuff, to the point that I started turning books upside down to read them when I wanted to slow myself down and absorb each word and sentence more carefully. Speed reading works best with textbooks which (IMHO) usually have extremely low information density.