From 1.5 hours before the performance, you can get student tickets (you need to be there in person and show a student ID, Swiss or otherwise). They cost CHF 22, and the seats are usually very good, often in the Parkett.
From 11:00 on the Monday of the week before the concert, tickets are available for CHF 20 (link).
This applies to young people up to the age of 25, members of klubZ up to 29, and students up to 30.