I suppose I had always taken that bit for granted, but it occurs to me that my first 4 years out of school was working for a Consulting Engineering Firm, so I was exposed from the start to the whole business development & proposal writing thing.

I'll have to go back and separate out what I actually learned during that period, since you're right that it's probably not entirely intuitive.

For the longer gigs, which made up most of my early contract work, it was mostly just a matter of telling them during the interview process that I'd prefer to work on a contract rather than salaried basis.