Hmm depends on what the people around you are like. I always book the bassinette seat a few weeks earlier than my flight leaves. Then you are pretty much guaranteed that its yours, unless someone with a younger baby books on the flight after you. Its never happened to me - I've had the front row for every single flight.
The short haul flights are the worst cause the front of economy where the bass seats are is usually where they put all the more "business" like people, just in behind business class.
So a few flights all I've managed to do is pi$$ off a pile of toffee nosed people in suits with no sense of humour. It totally sucked.....
Taking her on a train a few times might get her used to the whole idea of being confined but not confined to a seat if that makes sense? They wont let your baby crawl on the plane as its apparently illegal. Turbulence and all that jazz.