Well I would say ... very difficult, at least from my experience.
When I first arrived here last year, I spent 4 months, in average looked up more than 10 flats in the City. In the end, I got the current one via one of those expat website, who the person is kind enough to let me know they are moving out and they could hook me up with the landlady in renting their flat. I did and so here I am.
But then... unfortunately, my landlady wants the flat back for their fashion biz (they used the flat as their showroom before), so I have to move this year again.
Just yesterday, I visited a flat in Seefeld (the area I'm living now) 50m2, 2 rooms. When I arrived, there was already a long queue starting from the 1st floor to the ground lining up for visit already.. tough competition!
As said by Vishi, some factors can influence the decision
1) EU vs NON-EU vs Citizen --- I'm Asian, to be honest... I think that does affect it
2) Permit (L / B / C) --- I'm on Permit B ... don't see a problem
3) At times, if your employer is a popular one, it will help. --- Back in the time, I did hv my employer reference letter, but doesnt seem helping me a lot; but then, I have been told by a friend who work for Ernst & Young, she got her apartment and her landlord told her that it's because he knows the company and that it's a good one. So trust that's telling me ... maybe my company aint good enough? :-P
4) Get a letter in german from your employer that you are in Switz for the long haul --- As said I got my company letter and contract but well... anyway... should help to certain extend i guess.
5) Get a Swiss local to talk to the landlord right after you have applied and if you dont know swiss-german. --- I did... but my Swiss friend who actually talked to the property agent who told right in our face saying that... as a foreginer (me)... it wont be easy, as they would afraid foreigners will not pay the rent and run away.. and blah.... so to be basically, just an excuse.
Now... I could just wish myself luck in getting a new flat in the City.