I started the hobby with little money and big visions when I was 14, so I spent many hours thumbing through magazines and day dreaming. Little did I know that I was focusing my desires at the time so that, as the years passed, I had a pretty fair idea of what I wanted to do, so I lost little in the way of big mistakes and biting off more than I could chew.
If I were to start over, I would change only one thing, when the money was there to spend, I would resist buying it---whatever it was---until the next pay period. That way I could really think about where it fit into my ideas for my layout. It's a lot easier to sell off a boxcar or two at a loss than it is to sell off a whole fleet that just doesn't fit your idea for a layout, all because it looked cool at the time you bought it.