When I used to do web design in my 20's (I still do very sporadically), it felt like official business to schedule the "initial consult" and then to "sign a contract "with "my client". ;)
As a freelance writer and producer the waters were a bit muddier. I think the first time I gathered together my 7 or 8 1099's for my taxes I had my Sally-Field-at-the-Oscars moment: "Oh, I really have a business thing going here!"
As a non-profit director it was writing and preparing the tax-exempt paperwork to send to the IRS. Having to define and describe so many aspects of your organization in clear categories really forces you recognize that you have a full-fledged business happening. The letter from the IRS granting us 501c3 status was the cherry on top!