I really like what James of Finding Optimism has to say about clutter:

Simplifying is about streamlining your life; removing the distractions that aren’t important to you. By freeing up time, money, and energy, you can give more attention to staying well and to your highest priorities. If there is complexity in your life, then it will affect your ability to cope with depression. A good tonic is to simplify.

If you want to rationalize then the place to start is your goals and values. What is really important to you? How are you going to spend your allotted time on earth?
In my view “simplifying” is synonomous with “removing clutter”, and there are 3 kinds that I take to.
1. Physical clutter
When I buy something it owns me, not the other way around. My time, energy and effort goes into looking after it. What can I throw out instead? If this is you, then ask yourself what something will do for you every time you’re about to buy. Will it add to your life somehow? Or will it quickly become obsolete and a burden?
2. Lifestyle clutter
Are you committed to so many activities that you rush from one to the next, often running late and feeling stressed? Do you work back each night, bearing more and more of the workload? Do you find it hard to say no to new committments? Busyness is the new disease of the western world, but we, especially we, need to wake up and smell the roses. The stress of a rushed life is much harder for us mentally ill people to bear. We need to straighten our priorities, learn to say no, set reasonable schedules and not feel guilty about it.

Over the last few years I’ve learned the beauty and power of the word “No”. It’s OK to say no, and I’ve found that it actually garners respect. Everyone has to set boundaries on their life, but boundaries are only respected if you hold firmly to them. We need to focus on the most important things in our lives, like key relationships or spiritual growth.
3. Mental Clutter
I often have lots of things buzzing around in my head, but nothing gets my full attention, and nothing gets finished. (No doubt my love of strong, locally grown coffee is partly to blame.) When I start to feel overwhelmed I ask my wife to help me gain perspective. We work out the things that I actually need to do, not just things I’ve compulsively collected in a real or imagined to-do list over the previous fortnight. Some of the things are so menial, but they get put on a list and find a corner of my cluttered mind in a weak or manic moment. All of these have to go, so I can feel in control again. This has a wider application, as many depressed people have the same feeling of loss of control. I know that my very worst depressive episodes have been accompanied by this. Putting my mind into some kind of order, or decluttering, is a great help.