How do you purge unnecessary data from your brain?
What I do know is that the information I focus on is what stays at the forefront of my mind. And the more intently I focus on it, the more prominently it stays positioned (seemingly).
This is just me of course, but I try to organize my thinking like I organize my office. I keep the files I use regularly in my desk. I keep files that are important but not used regularly in my office but not in my desk.
Everything else, I store in file closets.
He said that the way to memorize anything and/or move things around in your memory is to make visual associations.
His name was Irv Wermont. He said, "Try to remember my name by associating it visually by picturing a little earthworm crawling around on my bald head."
It worked like a charm.
I find I can move memorized material around in my mind (from front to back and vice-versa) using that trick. When we move information to the front of our minds, other material is naturally moved aside (not perfectly, of course). If you memorized a page of MacBeth last week and memorize a page from Titus Andronicus this week, you'll find that MacBeth gets a little bit hazy.