train tickets: purchase how far in advance

Online, on the DB website ( https://www.bahn.com/en/view/index.shtml ) you will get large discounts for buying early.
For example, Nürnberg to Berlin, tickets for tomorrow are currently selling at €89.90 or €115.90, depending on which train. Tickets for mid June are selling between €24.90 and €79.90

Note, the discounted tickets are often only valid on the one train you select when you buy. You can't change your mind later.

Berlin-Prague: the price goes up the longer you wait but the cheaper tickets have the most restrictions
Salzburg to Munich: no need to buy these in advance; we took the Flixbus one way for 5 euros.
Munich to Nurenburg: the price goes up the longer you wait but the cheaper tickets have the most restrictions.
Nurenburg to Berlin: the price goes up the longer you wait but the cheaper tickets have the most restrictions

Salzburg to Munich: Buy the Bayern Ticket at the station on the day of travel and ride the direct Meridian train - €25/1 person or €31/2 persons for the 1.75-hr trip.

Munich to Nuremberg: Bayern Ticket again, buy at the station on the day of travel, ride the regional trains. The RE trains are fastest (about 1.75 hours.)

The Bayern ticket provides all-day inner-city transport on subways, buses, and trams once you reach your destinations .

You can pre-purchase the others online - the sooner the better if price matters.

For Salzburg to Munich:

The Flixbus website shows the fare in June from Salzburg HB to Munich ZOB as 7€, not 5€. The bus lets you off at Hackerbrücke ZOB, which is a ¾ mile walk to the Hbf or a 1,50€ S-Bahn or tram ride. There are five Flixbus connections between 9 AM and 8 PM.

In the same time period, there are eleven direct rail connections between Salzburg and Munich. They all go into the Hbf. If you use a 25€ Bayern-Ticket, you get unlimited use of the conveyance of the entire MVV (a 13€ value) in case your hotel is not at the ZOB or Hbf or you want to do further sightseeing in Munich. You can also use a Meridian GutenTag-Ticket for Salzburg to Munich Hbf direct, but you won't get use of the MVV. Or can go via a change in Mühldorf for 20€ with a Südostbayern -Ticket without use of the MVV.

For Berlin to Prague:

There are about 6 direct EC connections from Berlin to Prague via Dresden. They take about 4 hours and cost as little as 19,90€ with advance purchase.

So glad for this topic. Thank you!!

We want to go from Frankfurt to Berlin on April 27. When is best to buy? What are the options for kind of tickets? We are seniors.

skeaston, you wrote: "We want to go from Frankfurt to Berlin on April 27. When is best to buy?" - answer: yesterday, the cheap fares have already partially sold.
Go to the German Railways website: https://www.bahn.com/en/view/index.shtml
Enter, from: FRANKFURT(MAIN) (Note, there is more than one Frankfurt in Germany!), to: BERLIN
Date: Fri, 27.04.18, time: whatever you want. And click on "Search".

You will see lots of trains listed, with prices between €89.90 and €123.90. The cheapest tickets are already gone - sorry!
Reserving a seat is recommended (+€4.50)

When you buy the ticket will only be valid for the train listed. It will come as a PDF file attached to an e-mail. You must print out this PDF file and bring the print-out with you to show the ticket inspector on the train.

P.S. I am assuming you want to go from Frankfurt am Main city centre. If you want a train from the airport, the station is: Frankfurt(Main)Flugh

Thanks for all the helpful responses. :-)

German domestic train tickets are now among the earliest to go on sale, starting about 6 months (180 days) in advance of travel. International tickets and other countries usually have different rules.