C.R.A.P. (Clutter Reduction Action Plan)
Phase 1: Actual rubbish
Take a CAREFUL and thorough trip through every room and throw out rubbish. You'd be amazed at what you've put down in places because you can't think of what to do with it, and it really should have gone straight into the bin.
Phase 2: Stuff I Don't Need
Grab 4 boxes and go through the house and collect things to throw away (bigger things, not rubbish), things to donate to a new home, things to sell and lastly - things that you'd use if you painted it, fixed it etc. Set a time frame to do this (say, a month) and then work out whether these things are just hanging around and cluttering your life.
Phase 3: Reduce clothing hoard by 50%
Be ruthless and go through your clothes one sunny afternoon and reduce by 50%. This means looking at things with a critical eye - are they worn out? Are they out of fashion to the point where you won't wear them? Have you worn them in the past 2 years? I did this recently and was ruthless on tshirts that I love but are worn out, with clothes that are too small for me, and with things I've not worn since moving here 3 years ago. It was surprisingly easy. And means you wear everything in your wardrobe.
Phase 4: Hidden hoarding
Cupboards, boxes, bags, bookcases - all can be hidden hoard spots! Be ruthless.
Phase 5: Upgrade
Upgrade to higher quality, better planned systems/furniture that make for better organizing. Do this slowly and WITH PURPOSE so you don't get stuck again. For eg, I plan on upgrading our bookcases to tall ones to reduce their footprint... and hopefully use the books that I have on there!