Do a "virtual move" where you walk around the house - with boxes if you like - categorising things into whichever room is logical and pretend you're moving to a house boat or a tiny flat or abroad and trying to reduce the amount of stuff to take on the plane, so anything you don't need or love goes. Do a car boot or donate it but the important part - get it OUT asap or it will start creeping back into general usage again. Ebay is another option but only if you can trust yourself to be really organised with it - set aside a time once or twice a week to photograph 20-30 items, list and price, and to wrap up and despatch any items which have sold. Relist only once and any rejects, throw away or donate.
To build on Luv's theory, you need to identify the purpose, one by one, of every single item in the house. Two items fulfilling the same purpose - keep the one you like best. "This matches the cushions and looks nice" or "This reminds me of DD's first day at school" is an acceptable purpose, but not "One day I might find a place for this" or "This looked nice in the old kitchen". Again, knowing the purpose of an item helps you to identify where to keep it. Sentimental stuff and items that you use occasionally don't need to be in a drawer that you access regularly or (clothes etc) kept with your everyday stuff. Put it away, utilise the space which isn't easily accessible but where you can get to it if you need it or want to use it.
If you can take a couple of days off work to do the "virtual move" and/or get anybody in to help you, even better. If you live out of boxes for a few weeks while you reorganise, that's not an issue. It might give you a chance to touch up paint and/or deep clean behind things too if you want to.
For the more everyday stuff - once you've decluttered you'll still find problem areas which get full of "stuff". You want to identify what the problem actually is and solve that. For example I was always leaving wrappers and rubbish around the area that I sit on the computer. But putting a small wastepaper bin next to my desk solves this. Am I just being lazy? Yep. Could I walk to the bin to put my stuff in it? Of course. But the issue is that I don't do this. I think "I'm in the middle of something, I'll take it later" and then I forget. So instead of feeling bad about things like that, just look at what your actual pattern is and interrupt it. Toys left all over the place - buy one of those flexible trug things to chuck them in at the end of the day. Clothes discarded all over - can you keep a laundry basket in a discreet location? Shoes all over the hallway - buy a shoe rack, or one of those things you hang on the wall. If you're not using something, change the thing you're supposed to be using, not your habit.