You can start your search online before you arrive in Switzerland. Websites such as http://www.homegate.ch, http://www.immostreet.ch and http://www.immoscout24.ch are available in a range of languages (including English) and have a lot of adverts.
Once you are in Switzerland, you can also look for announcements in local magazines or newspapers which offer a selection of property adverts, such as La Tribune de Genève, Neue Zürcher Zeitung or L'Hebdo. However, these are only available in the region's language (French for Geneva and German for Zurich).
You may also wish to use a real estate agent who knows the market, will take care of all paperwork and deal with the landlord for you. Logic Immo, a company dealing with property in certain regions of Switzerland, offers a detailed list of real estate agents operating in each city of French-speaking Switzerland.
Real estate agents usually charge a registration fee of CHF 50, and then charge a second commission upon completion of their contract. Their exact commission should be stipulated (preferably in writing) before you hire them as you will be responsible to pay this fee.