I know friends who are always complaining about how untidy their houses are. I walk in their houses and realize why, everything is un-organised and nothing has a place.
Make storage boxes your best friend - Buy a few storage boxes and put specific items in them, typically things you don't use everyday such as books, magazines, CD's.
Keep your coffee table neat - The first thing I notice when I go into a persons lounge room is their coffee table. I'm not sure why, but I always notice if its neat or if there is just stuff all over it.
The only thing that should be on your coffee table is a box of tissues, remote controls, perhaps come coasters, and a centrepiece. Any more and it looks messy and cluttered.
Get rid of stuff on the floor - Always find yourself picking stuff up off the floor? I am always picking up cat toys from the floor. Keep a small box somewhere in the house full of your pets toys, and give them out to the pet one at a time. If you have messy children who leave their toys scattered across the floor, put the found toys in a box and confiscate them for a while.
Keep your lounge room neat but your couch neater - If your couch is messed up and it doesn't look neat at all, people will notice. Before people come over, prop the pillows in an attractive way and remove anything that shouldn't be on there. The amount of random stuff I have found behind pillows and fallen through the sides of the couch is crazy. I have found toys, blocks, $10 note, Mr Potato head eyes and a fork. Don't let it happen to you.
Dust constantly - I don't mean revolve your whole life around dusting, but dust is a very common thing to find in houses. Give the house a quick dust once a week or more, if it needs it. It'll make a difference. Also dust away cobwebs which are usually in the corner of the house or the ceiling.
Give everything its own home - If everything has its own place then your house will be a lot neater. Hats go on hat racks, jackets go on coat racks, games and toys are in the children's bedrooms and so on. Keep everything in its place at all times, so you don't get to the end of the week and there's jackets, socks, toys and Nintendo DS's sprawled across the house.
I was always taught...
A place for everything and...
Everything in it's place.
So simple and it works....well until the wife gets home from work
She gives me something to do when she leaves for work each morning.
I can put everything back in its place, ready for those random visitors.
The reason things build up on surfaces is because those people don't mind it and don't notice it the way you do. They become blind to it. This is just a non helpful post saying, "why can't you be more like me". As for finding things between couch cushions. What were your hands doing down there? Perhaps your ability to notice the shortcomings in other people's lack of cleaning habits means you should take up housekeeping professionally so tha those that have better things to do can do so then you can be happy too :)
You clearly don't have toddlers! Also, usually when someone goes visiting they do it to see the person not rate how clean the house is - I would hate to have someone like you visiting with such a shallow view of the world!