Loco2.com is my favorite place to buy European train tickets. Although it doesn't cover every European destination, it allows you to book the same tickets you can purchase on British, French, Italian, and German train websites--and it's easy.
Loco2 recently shared a post on how to buy cheap train tickets in Europe that I think is well worth reading for anyone planning a European train vacation. What's more, it gives you the opportunity to set alerts on their site to notify you when cheap tickets come available. Bookmark the site and get on board!