Sometimes it's easy to see when you have too much. If you can't see the floor in your room, then yes, it's time for some things to go. But what if you are a reasonably neat and orderly person? You still need a system in place for getting rid of the few things you no longer need.
Systems are what keeps organizing people in our happy place.
Without systems, that old shirt/game/book that you want to part with ends up on the dining room table for a month. Or it gets shoved into a random closet to be found later on during a cleaning purge. Without systems, no one knows exactly where things go and all that random stuff ends up in a junk room or the garage. Without systems, you will always hear the cry of, "Mom! Where is my _____ (insert randomness here)?"
The easiest way to solve this dilemma is to adopt the Empty Box Habit. Place an empty box in the bottom of your closet and label it "Donate." When you get dressed and try on something that just doesn't flatter you any more or has gotten too big or too small, immediately put it in the box. Don't fling it on the floor, the bed, or hang it back up to try again at a later date.
You will soon find that once you place one item in the box, you are more apt to keep going. If you realize that you regret some of your donation choices, well don't worry because it takes a while to fill the box. You can always unearth something a week later if you change your mind. However, if a month has gone by and you didn't pull out anything, then you probably don't need it anymore. So yes, it's time to safely donate without regret. Place your box by the front door, take it to the car on your next trip, donate all your items, and then repeat with a fresh new box.
This approach works very well in closets, but you can also stow an empty donate box in the garage for the random toys, books, dishes, and more that don't belong in your house any longer. Think about it - everyone knows where the forks go in your house and they don't bother mom about it, right? If everyone knows where the donation box is, then they will just start loading it up as need be.
If you have little, rapidly growing children in your family, then a box in everyone's closet is a must have. If you are saving clothes from your oldest, to give eventually to the youngest, then put 2 empty boxes in their closet. Label one box "donate" for toys, clothes they dislike, books, etc. Label the second box with the current size of their clothes, since you will not be donating these, but rather reusing in a few years. When the hand-me-down clothes box is full, put a lid on it, place it in the younger child's closet, and put a fresh new box back for the next round of clothes.
Go try it out today - your closet will thank you.