By the same token, you shouldn't undercut your competitive advantage of having done past work for them. If they can get the most efficient service and answers by approaching you instead of someone else, you should be compensated for that and it still works in their favor.

In the legal arena there are some structures in place to help this happen. Clients are used to paying for services on an as-used basis, so that expectation is there. With a past or prospective client you can say that you are happy to provide some quick guidance but to give specific answers would be legal advice and they would have to be a current client (which may require a conflicts check, letter of engagement, etc). On the client invoice I frequently list time spent but not billed so they can see the extra value they are getting even though they are being charged for some items.

The key is client expectations, part of which may be industry standards and part of which you establish with them. You don't want to alienate someone who makes the hiring decisions, but at least find a way to point out the value they are receiving even if it's an invoice of all no-charge items. It's also true that some clients are better than others, and getting to a point where you can be picky about who to work for is a good place to be.