Many of the posts here are about decluttering: getting rid of things that are not useful, beautiful, or do not fit well in your home.
The first step, really, should be not to let the stuff come in.
It is well worth your time to go through your email and unsubscribe from lists you are not interested in. It takes a bit more time to unsubscribe from paper mail. Wherever you can, tell them no. No. Do not send me offers. Do not share my name with your associates. Do not send me catalogues just because I made one purchase.
Be strong when you are at an event such as a conference, the Home Show, or a fair where people are handing out “free” stuff. Free is very tempting. Ask yourself first if you would pay money for that freebie. If the answer is no (are you kidding? for that piece of junk?) don’t cart it home. You don’t really want it. No, thank you. Walk on by.
Unless you truly need something, do not look on the free section of Craigslist. Do not surf Freecycle. Do not stop at garage sales. Often, people are offering stuff for free because they want to avoid the cost and trouble of dragging it to the dump. Do not become their dump.