At recent writing events and online, I've witnessed several writers claim that the future of making money writing will be through writing a lot of thin slices of content covering everything. These writers claim that the way for freelancers to move ahead is by spreading themselves out horizontally writing on several different topics, making them as much an expert on cooking as on the automotive industry or (insert subject here). I may be in the minority on this, but I don't think becoming a generalist is the way to freelancing success.
First, freelancing is a business, and the most successful businesses specialize. Law firms specialize in particular fields of law. Top shelf restaurants specialize in certain dishes. Why should writers be any different? Sure, I think it's fine for writers to offer multiple services, but they should really try to focus on those few niches to make themselves "go to" writers on those topics.
Second, freelancers who write on any and every thing run the risk of turning their finished products into a fast food or Wal-Mart type product. The writing could be great, but since you're offering everything and are an expert in nothing, your chief weapon of choice will be to offer lower prices. This means the generalist writer is spending more time working for less money, while the "go to" writer has more room to negotiate better rates and terms.
At the end of the day, I feel writers should brand themselves as experts. It's important for freelancers to know who they are trying to be and what their target audience wants. If you're chasing after everything, you might make some money, but you're going to work yourself to death in the process. There's a lot of work involved in branding yourself, but eventually, the work will come to you, and you may even find yourself turning down assignments that aren't "right for you."
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While it's not specifically geared toward freelance writers, Seth Godin wrote a great book on this topic called Linchpin, in which Godin speaks to the importance of making yourself indispensable.