Consultation versus estimate
When you go to the doctor, do you expect him to see you for free in hopes of selling you some prescription? Of course not. And if he did, would you really trust his opinion, or would you suspect he would be trying to push some drugs on you that you really don't need?
Here's how I do it: If you know exactly what you want, you get a free estimate. If you don't know what you want, what bug you have, or you want to know if your tree will fall on your house, then you need a consult. I do the consults myself, and I charge for them. Most of the estimates are performed by others, and that eliminates people trying to get free information. Whenever I do the estimates, I make it clear that I am there to do an estimate, but a consultation is available for a fee. When people still try to beat the system, I stick to my guns. Most people get it, but occasionally I get someone that says they don't need a consult, but when I get there, the first question they ask is something to the tune of, "What do you think of this tree?"
I am more than okay with someone picking my brain for advice and information as long as I am being paid for my time. Then, I would love to get more work from the clients, but they can hire someone else or do the work themselves, but I am okay either way.