This is so true, I did this when I started charging for community management.
The first few ones I did were free and give me anything you want.
But when I got better at it and started to charge as a standalone service, I needed to make that clear.
Always let them know.
When someone asks for a favour that involves you using your skill for free, be clear about the time you can give to it.
If you're really close to them, tell them you'll be able to do it but you'll need more time.
If you're not so close or you honestly don't want to do it because it'll take too much from you, apologise and decline.
On both occasions, always let them know how much it'll usually cost if it's a paid service... It gives them perspective of the value.