Eurail Pass reservations

I'm worried about good train seats (esp. overnight sleepers) selling out. I didn't think about it before, but I can't make reservations until my rail pass gets activated, and I can't activate it until I'm at a train station in Europe. So that sucks. I wanted to start booking some trips. Oh well. This will help me get out of the habit of having everything planned ahead of time. Just wing it and be OK with it. Train travel isn't like plane travel here in the US. Point for trains.