Marie Kondo's book The Life Changing Magic of Tidying Up is absolutely worth it's weight in gold. I'll be honest, when I finished the book, I felt equal parts overwhelmed and motivated to go through my things and start to clear out the clutter. I've recently lost a bunch of weight and haven't gone though to clean out my closet of items that don't quite fit me anymore, but this book gave me a good starting place.
Kondo lays out what she terms the KonMari method of going through your house item by item and getting rid of stuff. In short, she suggests that you shift from the mindset of "what should I get rid of" to "what should I keep," and you should only keep those items that truly bring you joy. If you're indifferent about an item or are keeping it out of guilt, it needs to go.
Some of her methods were a little extreme and felt kind of silly as I acted them out in my head, but her basic principle of only keeping the things that actively add positivity to your life is a good one. She has an app out now that walks you through the parts of your life from clothes to books to keepsakes to help you make a schedule and stick to it.
I highly recommend her book to help everyone de-clutter their homes and reduce the junk that piles up after years of letting it go.