Remember to reserve sleeping accommodations in advance. Start your Eurail travel in the evening. Read the small print on your pass for the details, or ask your travel agent for clarification. Bring your own food and drink, and lots of bottled water. Dining facilities on trains are expensive, and you shouldn't drink the water from the bathroom taps.
Youth passes and Saverpasses often offer substantial savings.
Protect your belongings by locking them to the overhead rack, especially on night trains. A cable bicycle lock and a padlock are ideal for this. Pack as light as possible. It's easy to get around by train with a single bag, but it gets much harder if you have loads of luggage, especially if you arrive at a station without a baggage cart in sight. If you are traveling on a Flexi-pass, don't waste a day of your pass if your journey is just going to take an hour or two.