I did something like this about 10 years ago, it’s possible but it’s a demanding business.
It’s doable, but it will take some time to really find your client base. The big marketing problem is that the companies which really need your help are often the least likely to be able to recognize the fact that they need your help. Smaller outfits – particularly those which are geographically isolated and out of touch with industry best practices – and indie studios without much production history are the best clients for this kind of work; they are also the least likely to realize that it’s what they need and often they will be reluctant to pay for what they really need.
Often consulting business work best if you have a couple of reliable clients – typically people you already know from past work – who can provide a little stability and help spread the word. It company X will keep you busy for a few months out of the year you’ll be able to work on finding new clients without worrying too much about paying your bills.
The first few jobs are the hardest. Once you get going and get some word-of-mouth advertising it becomes much easier. It helps if you can get clients in hub areas where there are several companies close by. Since people move from job to job within those areas your reputation will spread faster there. Ads cost money, and are very inefficient; in a small industry like ours a good reputation and good contacts are far more valuable.
A lot of work in this kind of thing goes into just drumming up new business. Expect to talk to ten companies for every job (at least!). Any time when you don’t have a contract you’ll need to be out trying to get meetings. Giving talks at shows like GDC or Gamescon is a good, so is writing articles for Gamasutra or Develop. The big thing is to go out and look actively for business; even in the age of the internet its not likely to come looking for you.