What Urwumpe said, basically. People don't go to the gym and lift weights so that they can then go out into the "real" world and use their training to lift other bars with heavy objects on the end, they go to the gym and lift weights to build their muscles up which can then be used for whatever you can use muscles for.
Same thing with intelligence. Use it, it develops. Even if you fail or make a mistake (rather, ESPECIALLY if you fail or make a mistake) it develops.
One key tactic I use is to 'check my head' whenever I'm considering something, i.e. take an inventory of my assumptions. Then, I assume that all of my assumptions can't be taken for granted, and I explore a problem or concept without taking my assumptions for granted. That allows me to look for and ask questions that I might not think to ask if I was taking all of my assumptions on faith.
I think Einstein was right about imagination and intelligence. Or rather, I think that imagination + memory is effectively equivalent to describing human intelligence, as if a human can't imagine something, they probably can't describe it or work with it very well. Consider a person (we'll call this person 'A') who calls another person (we'll call this person 'B') for advice on a problem with their car. Person B happens to be a car mechanic, person A is not. Person B can't see or touch the car engine, and person A has no idea what they're talking about. The only way person B can help at all here is if they can imagine the car, the possible problems with the car and also how to phrase questions that person A can effectively address (without having any useful knowledge about the subject).
It's far too early in the day, and I've have far too little sleep to be responding to a post like this ...
