Inform them, that you have not set any finish date because client work comes first. If anything important comes up, their project will be postponed. It is quite polite, if you play it right, they might understand the situation without problems. This will also make them more likely to pay for your service ( you can offer a small discount ).

Regards,
Radek

One easy way to squash free work requests from friends is, make them do all the pre-project groundwork — gather pictures, do the necessary research, etc — in the same effect of what Matt Keegan mentions above — show them it’s not a simple process. I don’t ever have to say “no”, and I’ve found that they never mention it again.

There’s a danger to the general “I rub your back, you rub my back” policy that Amelia Sauter story sort of encompasses in her comments above. Sometimes, someone can’t repay a favor to a friend who’s helped them when that friend wants it. It’s happened to me, and nearly destroyed a friendship. I prefer that either money changes hands, or it’s known from the start that it’s free — no “favors” — because the gray area is hard to navigate.