You can book with DB Bahn............
http://www.bahn.de/international/view/en/index.shtml
on their website up to 3 months prior to departure. They have some really inexpensive fares,they usually go fast. I got mine for 19euros. You can choose from 1st or 2nd class, 2nd class is fine unless the 1st class is just a few Euros more. You can set up an account even if you won't be using it much, and print your ticket right on-line. Also if you are travelling to Germany from another country it works also, but then a paper ticket will be mailed to you. I have booked many tickets this way.
Excuse me, but if you want to correct me, correct me the right way: Accoording to the website you CANNOT book the WAW specials online:
nicht im Internet als Online-Ticket und nicht am Automaten, nur im personenbedienten Verkauf in DB Reisezentren, DB Agenturen und im DB Reiseservice erhältlich.
Offers to WAW are only available over the counter. You can book the regular fare online, which does no male an aweful lot of a difference.