I assisted for a fair while then broke out and moved to paris to put together a book (all this after dropping out of uni), took out a loan, bought loads of kit, spent long winters trapsing around magazines doing appointments, coming up with ideas, going insane...
There's no particular route I don't think. I tried to go a bit die hard with breaking through, but probably lost my early twenties because of that.
Going to see and pester agents is wrong though, I find if an agency is good, is on the pulse and likes what you do, they will come to you.
An agent is only as good as their clients. Once you start with an agency you are no longer in total control of your career and there are numerous people editing your books and painting the picture of how clients see you. That is not always a good thing either.
For every plus point of having an agent, there is a minus.