Once in a while (for some once a year) you clean out your house to try and get everything back in order, in a state of perfection. You do this by getting rid of all the superfluous clutter and junk. But what if you don't know for sure something is superfluous or junk? This is where the bag comes in.
When you have something and you don't know for sure if you need it or if it's just junk, you take a big plastic non-see-through bag (it can also be a box) and put everything you're doubting about in it. Once the bag is full, or you are done cleaning, take some duct tape an close the bag. Write a date on it for six months or one year in the future and put it in your garage, the attic or anywhere else you can store this for a while.
After those six months or a year, take the bag and throw it out; without looking whats inside. If you did need something, you will have probably taken it out by now.
It a simple idea, and probably been told many times, but it works.
Quick tip: write down everything you put in the bag, this way when you are searching for something you need and don't know where it is, you can always quickly check if it's in the bag.
Why use duct tape? This way it will take some effort getting into the bag and you won't be lured by it and just go through it all and get something out.