Then start charging.
The next time someone asks you to "help" them say that you'd be happy to, but it's actually become more of a recruiting role, not just passing info along to your friends. Then have a fee structure ready. Make it enough to be of value to you, and reasonable enough to seem like a great deal to the potential client. That's how you start to build a business line.
For instance, figure out the amount of time it would take you to do a search on their behalf, screen, interview, etc... Then use your overtime rate as a starting point and multiply that by the number of hours you think it would require. That will give you at least a starting point to think about fees.
If I'm the potential client, I'll then want to know if you offer the other things agencies do - full background checks, reference checks, guarantee of the person such that if they don't work out in the first 90 days (or six months or whatever) that you'll rerun the search for them free of charge, assurances that you're not being paid by applicants in any way, etc...
There's probably much more that you should think about also, but as a pp said - if you take this on as a more formal arrangement it carried with it expectations and liabilities that you should think through carefully.