I don't know how everyone else does it but I have a one freebie rule.
If I go to see a prospect and don't get the business - that's the last time I visit for free.
The percentage of times those people will contact me again -- usually 5+ years later -- is oddly high.
I tell them - one initial phone conference is fine (I can limit the time I spend) however anything else on-site is fee based.
Guess what ... wait for it ... wait for it.....
Most disappear again once they learn that there is a possible fee involved..
And I save myself four or five hours of travel/meet time (during which they're usually trying to gather as much free advice as possible)