I simply wouldn't do it and, in the worst case, drop the lead. Asking for a free trial only shows that they do not value your work and that is the worst place to start a relationship from.

This hasn't happened to me, but if it did... I'd simply say something short and sweet like... "Thanks for the interest, but I'm not doing pro bono work at this time. I'll let you know if I hear from anyone that is. Have a great day."

I'd draw a very clear line that this is not something you can do. If you accept it, the client might try to pull all kinds of other tricks because they might now you're willing to give in for the sake of keeping the relationship. Don't be afraid of losing a lead (unless you are in a very desperate situation or get some other strategic benefit from this).

They need to decide whether they want to hire you based on your conversations and your portfolio of prior work. That's how business works. Try asking a restaurant for a free meal with the promise you'll come back if you like it.

Something along the lines of...

"I prefer to work with clients who respect the value my business offers and the time/skill it takes to produce quality work. As a sign of respect and a show of good faith for the future, I'd be glad to do a small trial project at my hourly rate."

More on this:

http://www.followthegospel.com/post/24146731512/startups-quit-asking-for-spec-work

A firm "no" is usually enough. I wouldn't care about losing the lead if that's how they want to start the relationship.

Thanks for your comments, everyone! Helped me a lot.

Next time politely say, Sorry that's not how I work.

Your time is valuable. If the client can't decide whether they want to work with you from your portfolio, website, case studies, etc, they are not a client you want to work with.

Your work adds value to the client. Behave like it.

No is the good word.