Have a plan
Specify a time when you will work on a designated area of clutter and don't stop until it is dejunked. Start gradually, with one cupboard or corner of a room.
Mementoes
Don't feel you have to throw away objects with sentimental value, but chuck anything with negative emotions attached.
Take it easy
If you are unsure about getting rid of something, put it in a box, label it and store it. If you haven't looked at the contents six months later, throw it away.
Maintenance
Dejunking never ends. Develop good anti-clutter techniques such as opening mail next to a rubbish bin, and replying immediately to letters, emails etcetera.