You need to pretend as though you are moving house (or country!) and be really ruthless. Was the best thing I ever did - had to get our possessions down into 16 boxes.
Do it properly, right down to ordering/borrowing moving boxes and taking a couple of days off work when the kids are at school/finding someone to look after them. You can get boxes for £20 on amazon. Then go through every room, packing stuff up, you ONLY keep it if it has a purpose, something it's actually useful for - so not something you've used once in five years, not something which has a tenuous sentimental link, not something which you bought because you liked it but it really doesn't fit anywhere in the house. "Useful" can also mean "makes the living room look nice" or "Helps me remember when DS was a baby". Start with cupboards - it's much easier than trying to sort through stuff which you use every day but you need to work up to that. Pack every last teaspoon!
You have to be really ruthless. Don't keep two things which fulfil the same purpose unless you actually need two.
Once you've cut everything down, you know what the purpose of everything in the house is, which means it's easier to find a logical home for it. In addition, everything is in boxes. You can do a proper deep clean of the house and/or repaint any rooms that need it. Now you can go through the boxes and unpack everything but make sure that where it goes is logical - if the purpose is related to cleaning, designate a cleaning area. If something is used mainly in the living room, store it in or near the living room.
Then like Contented said, set up systems which work. If you have clothes lying around the house, put smaller laundry baskets around for clothes to be put into rather than left. Get a place for paperwork to go if you can't deal with it straight away or set aside a time of day/week to sort it. Sort recycling as you go rather than sorting it out later. Etc. Don't try and work against your nature thinking "I will remember to take all of the dirty clothes up to the laundry basket" just work your actual patterns into something livable and then don't beat yourself up about it! If you give children easily achievable goals too, they're more likely to do them and you can increase it later if you have the inclination.