You just need to sign a simple contract with the buyer, usually one page. You can find a template at most "garages" and possibly online. You could even do it in handwriting as long as it has all the elements to correctly identify the car, price, seller and buyer. You then hand the "carte grise" to the buyer and with it they can go register the car on their name and have new license plates issued.
If you are buying another car you can keep the number plates, otherwise you should return them to Bureau des Automobiles.
As a precaution I would recommend to hold on to the "carte grise" until you have received the payment. Although it usually sounds bizzare to foreigners, it is customary in Switzerland to request cash payment when selling a car 
One more thing, instead of selling the car yourself you could mandate a "garage" to do it. They will handle all the advertising, talk to potential customers and handle the paperwork for you for a % / fee. For the buyer a garage can make the deal more appealing because they usually offer 3-6 months warranty and financing plans.
Sep 22, 12 17:14