If you tend to horde junk, and you know who you are, consider launching an all out attack on your cluttered lifestyle. The best way to get the ball rolling is to Schedule a Cleaning day. Plan to spend the entire day cleaning and throwing things away, without interruption. A good rule of thumb is to think big. Don't spend the day dusting nick-knacks, and sorting file cabinets. Go for the jobs with the most impact. Decide to finally park your car in the garage, or to completely empty a bedroom and starting over from a blank slate. Remember the goal is to throw out as much as possible, otherwise you are just moving junk from one part of the house to another, in a wasted effort.
If you are wondering whether or not to keep an item, odds are you should just get rid of it. Try to understand that you subconsciously hold a higher unrealistic value for your belongings. If you honestly believe that an item is worth taking the time to sell, than stop what you're doing immediately and go sell it. Posting something on Craigslist should take no more than 10 minutes. If it takes you more than 10 minutes, than you are wasting time, and as punishment you should sacrifice the item to the trash for avoiding work.
Renting a dumpster is another good way to force yourself to throw out Junk. Its the "American Way" that if you rent a dumpster, you are going to fill it to the brim because it's paid for, and we want our moneys worth.
Work at a face pace and be quick to make decisions. Your first instincts are usually right. It's only when you stop and think that your brain has time to coerce you into keeping stuff.
After a full day of clutter carnage, sit back and enjoy the fruits of your labor. You will feel at peace and relieved from a burden you never even knew you had. After I simplified my life and got rid of my junk, it astounding to me that people can live any other way.