Swiss Half Fare Pass

Yes, you must buy a ticket before boarding. If it is a vending machine, just indicate that when you buy it. It will charge you the Half Fare price and print out your ticket showing it is half fare. If the conductor asks to check your ticket, just show him the ticket and your Half Fare Card and ID. Then he will know you paid the correct fare and will be on his way.

Jeff, Yes, you have to buy a ticket before you board the train. No, you do not have to go to the ticket counter, you just buy it from a ticket machine, which will save time. When you get to the end of the transaction, it will offer you two options: "Without Half-fare-card" and "Half-fare-card / Children".
As Sam says, when you encounter a ticket inspector on the train, you have to show both your ½price ticket and your Half fare pass.

The same applies for urban transport (trams and buses), buy a ticket from a machine before getting on board.
For rural buses you buy from the driver, and you have to show him your Half Fare Pass. Say: "Luzern bitte, mit Halbtax".

There is an online simulation of using an SBB ticket machine here: http://www.sbb.ch/en/station-services/am-bahnhof/dhl-service-point/automatic-ticket-machine/online-demo-billettautomat-sbb.html

@Chris F -- do you know whether US-based chip-and-signature credit cards will work in these machines ??

Laura, sorry, I don't know. I have a Swiss debit card which works, and I know debit and credit cards from other European countries work, but I have never asked a US tourist whether their cards worked.
There is some information here: http://www.sbb.ch/en/station-services/am-bahnhof/dhl-service-point/automatic-ticket-machine.html
Which says "Contactless, faster payment with the Postfinance-Card and the American-Express, Mastercard PayPass and VISA payWave credit cards", which sort of implies US cards would work.

You can but try, and if it doesn't use old fashioned cash. All SBB machines take both Franks and Euros, notes and coins.

If you do use a "foreign" card in a ticket machine, DO NOT get caught by the dreaded DCC. If it asks "do you want to pay in your own currency", the answer is NO, pay in Franks. I have seen people get caught by that at Zürich airport.

Thanks, Chris.