For each of us, our surroundings have a great bearing on our mood and mental state. Whether it is your bedroom, office, bathroom, kitchen, closet, or living room, it is important to keep the places you frequent clean and organized. If you are in a room that is cluttered and messy, you are going to feel more overwhelmed, scatterbrained, and chaotic than if the room was more under control.

Organize
Perhaps the simplest and often fastest way to make a living space more livable is to organize it. Get clothes off the floor and into drawers and onto hangers, put books back on shelves, sort scattered papers and put them where they need to be. If something doesn't belong in the room you are working on, either take it away, or put it in a box/bin of things to move. A lot of these things sound like no-brainers, but sometimes when an area gets too cluttered, it can be difficult to separate the different tasks that need to be completed.
Use Chunking
One way to chart your progress is to work on one task at a time. Break up your cleaning into smaller “chunks” that aren't as overwhelming. As you carry out these tasks, you will see the area around you become more and more manageable as you go. Chunking can be a useful tool for other things, too, such as working on a project, studying, writing, or even working out.
Get Rid of Stuff
Sometimes, we just have too much stuff. Comedian George Carlin did a great bit on “Stuff”, which can be seen here. There is a difference between the stuff we need, and the stuff we don't. While that sounds obvious, it's never that simple. There are many things that we hold onto for various reasons, and have difficulty giving away: we think we will need it some day, it brings back memories, it was given to us as a gift, etc. With these things, really think about it. Will I ever read this book again? Watch this movie? Will I ever get around to hanging up this poster? If the answer is no, or at least “probably not”, then go ahead and get rid of it.
There are more great tips for dealing with stuff on this blog.
Keep it Clean
Once you have finally organized, cleaned, and hauled all your unwanted stuff out of an area, the last and sometimes most difficult step is to keep it clean. To do this, you must continuously remind yourself why it is important for the area to be clean, and sustain motivation to keep it that way. Things have a tendency of piling up quickly so weekly, daily, go through and clean things. This will probably only take a few minutes, but if done regularly will save you hours upon hours of cleaning down the road. Also, try not to buy things that you really don't need. Before getting something, think to yourself “What will I do with this?”, “Where will I put this”, and most importantly, “Will I actually use this?”.
Best of luck on your path to a less cluttered living space!