We mainly accumulate so much clutter for two reasons. First, we don't know what to do with something when we get it, we don't know where to put it, so instead of figuring that out we just throw it wherever. Repeat a couple of times a day, and it's no wonder we have so many loose piles of stuff cluttering up our lives.
The second reason we have so much clutter is because most people are terrible at throwing things out. We accumulate things we need for largely emotional reasons, whether its sentimentality over the item or anxiety over needing it and not having it. While we're very good at rationalizing why we need to keep something useless, we aren't so good at coming to terms with the fact we just don't need it all.
Generally, we use a couple of stand by reasons why we don't throw things out. We think that we might need it someday and it's worth keeping around until that indeterminate time. We feel sentimental because we received it either from someone special at a certain point in our lives, and we keep it even though it's long past its usefulness. We keep things because we feel that the item might be valuable, and someday someone else might want it. We keep things because they may not be used, but they aren't exactly broken yet. Or if it is broken, it can be easily fixed. Or it'll be useful again if some life changes occur (like weight loss or a move). Or you can give it to your kids or grandchildren, even if you don't currently have any kids or grandchildren. Or you plan on using it someday, even though you haven't in the five years you've owned the item.
While all of these excuses might contain some modicum of honesty or practicality, when it comes down to it they are just excuses. Even those potentially valuable items that you might use some day, like books, just obviously have never been a real priority. For some items, it's not whether they're useful- it's whether they're more useful than what you already have or employ.
Asking yourself each item's worth relative to what you've already got going on in your life is a great way to start prioritizing and eliminating some of your junk. Some other good questions to ask yourself are: do I really want to keep this item and wait to see if I'll actually use it in the next 5, 10 or more years? If I was forced to eliminate most of my stuff, would this item make the cut? Do I really have anything serious to lose by throwing this out?
Most of the time, after asking yourself these questions, you'll at least rationally realize that most of the items under evaluation just aren't worth cluttering up your life with.
All that leaves are the sentimental items, which are often the hardest to get rid of. While it's best to evaluate these items on a case by case basis, but there is one good rule of thumb to consider when you consider keeping these items around. Ask yourself: am I keeping this item around because I can't let go of it, or because it legitimately enriches and improves my life?