Time is money, that's a simple, irrefutable fact. That said, I get a thrill out of people asking to pick my brain; it means that, at least in their eyes, I have developed skills, expertise and knowledge in an area that they themselves are exploring and I have the ability to get them using a GPS rather than an early 20th century compass.
Going back to the time is money thing, I usually evaluate the "can I pick your brain?" people by asking myself: "is this a one-offer or is this person going to keep coming back?" One-offers are generally just seeking reassurance that they're doing the right thing. I have a lot of start-up folks come to me to "pick my brain" and really what they're saying is, "Please tell me I was right to leave my $40,000 a year job to do this." If I get the feeling that that is the case, and the person just needs a pep talk, I'll make time for them after hours when I can sit back, have a beer and chat casually.
The ones that are going to keep coming back aren't looking to just pick my brain, they're looking for a mentor. They're going to be looking for reassurance plus guidance and maybe a little bit of a push in the right direction because they've worn their ideas thin. I do talk with them once or twice but beyond that, I'm straightforward and say, "Look, I've been where you are, I know it's tough and I'm happy to help you but realize that I do what I'm doing for you, for a living. I have to make money at some point. I'm glad to keep going with you but I'll have to invoice you for the time." And that's not to make them feel bad or for you to sound like a jerk, that's to outline very clearly that your time is valuable, just as much as your advice is.
Don't feel like you're sounding like a jerk by stating clearly what you need to get out of a situation. That's business.
Good luck and feel free to get in touch if you need to follow-up!