It depends on the friends of course, but I usually invite myself over for dinner, then fix their issues afterwards. For others, I usually send them a bill of materials, with the strong implication that those costs are theirs to pay. Or I do both.

Others have already mentioned it, but for companies, the first response should probably be mentioning your hourly rate: "Sure, I usually work for €80/hour, let's setup a meeting to discuss what you need done". Such pre-sales meetings (including travel costs) are usually at your own expense, so you need to factor those in to your hourly rates.

Teaching can actually be a good starting point as well, both for network building and to assess their skills should you need them in the future. Still, remember that good teachers don't solve their students' problems for them: they only give pointers and explain concepts. If they ask you to do their work for them, you can turn it around and confront them with it: "I thought you wanted to learn? Of course, I can do it for you as well, but in that case I'll need to charge X/hour".