Here are The Professor's best tips for traveling by high-speed rail in Europe:

Buy Tickets Early:  The best deals on trains can often be found when booking early.  For instance, in Italy, we have found that special deals in first or business class can be had for a lower price than traveling in standard class.  Also, travel on holidays or peak days (Sundays in particular) can be sold out in advance.  It is always better to book early.  This is probably the best tip for traveling by high-speed train in Europe.

Don't Buy Reserved High-Speed Rail Tickets for Short Trips:   For instance, if traveling from Venice to Padua (about 25 miles), it is about $18 to ride on the high-speed train getting you there in 28 minutes.  However, the cheaper ($5.25) regional train gets you there in 44 minutes, sometimes even faster.

Buy Tickets from Trainline.Com:   The Professor finds this site/app to be the one-stop place to buy tickets in the UK, Austria, France, for Eurostar (chunnel), Spain, and about 40 other countries.  There is a small service fee but the convenience and ease of use is worth it.  Trainline does not serve train service in Norway, for example.  

Select Seats Near the Exits:  Sitting in the middle of the train can be kind of chaotic when trying to drag/grab luggage to get on/off the train.

Know Where Your Car is Before Boarding the Train:  Read the ticket to see which car (and seat) you have.  Look at the map posted on the platform to see where that car will actually stop at the platform.   That avoids the "tourist sprint" to get on the train.

Let Everyone Off the Train Before Getting On:  Avoid that logjam at the train door.  Be patient.  You won't miss the train.

Bring a Picnic Lunch (or Dinner):  Unless traveling business/first class, don't expect much more than a beverage cart serving drinks and day-old sandwiches.  The scenery is beautiful, enjoy it with some luxury and local cheeses, a fresh sandwich, a little bit of wine purchased right before departure.  Most trains do have a bar car with food and beverage, but they can be crowded and have limited choices. 

Join The Traveling Professor on a small group tour to Europe, many feature scenic train rides.