I have some potential clients that ask to see something before accept the work. Now, should I make, for example, a free mockup or It's better to find another client?
That sounds like Spec-Work.
You should show them what you are capable of (i.e. your portfolio) but be wary about doing any work for them for free.
You should have a portfolio - pieces that show examples of the type of work you are able to do, the styles you work in, etc. If they're asking for a "free sample" of the page make sure you have a legal document in place to protect your work (and copyright your work before you sent them the "free sample").
There are plenty of legit people that can't imagine a concept well enough and need some kind of visual hint, but there are also plenty of evil little scheming people who want something for nothing (or who don't feel like design is real "work"). CYA.
Stand your ground. Don't do SPEC work.
The client is interested in you (based on your portfolio, or word-of-mouth) - capitalize on that.
Offer "X" comps for the job AFTER the contract has been signed. That way you're both protected. They get their comps, you get the work (and paid). And no one will feel cheated, or slighted in the exchange.
The only time that comparing actual products makes sense is when you are buying something that is essentially pre-fabricated, like a subway sandwich where they ask a few questions about what you want and drop a few swappable pieces into a pre-prepared formula.
If this is your business model - if you bash out a lot of very similar designs quickly for people happy with something that isn't a great fit but does just about work - then spec work and mockups make sense. But this is the only time that they make sense.
If people want serious design work that really fits their need they need to understand how the design process works. Here's a classic diagram by Damien Newman:
If you're doing spec work, you would be going all the way through the squiggle and half way through the straight line before you even know you're doing the work: rushing through all the hard stuff and leaving only the final polishing. That's not just bad for you: you'll be cutting out so much of the important squiggle at the start that it's more or less impossible that you produce something that matches the client's need more than superficially.
Realistically, you're going to be recycling more or less the same concept as last time, with a few tweaks that are the equivelent of swapping tomato and mayo for peppers and chilli on a meatball sandwich.
The work will be average, but plenty of designers do make a reasonable living like this. There are plenty of worse ways to make a living.
For something genuinely good, however, that genuinely meets the need, it's never going to work. The majority of the work, time and skill comes in before the concept line: really understanding the context, and developing, testing and improving ideas based on a deep understanding of the client's actual need.
There are plenty of clients who are happy with average but professional-looking work that sort of fits their need even though really it was designed for someone else. There are plenty of middle-of-the-road designers who are happy to churn this stuff out. If that's not you, and if you think that's not really what they want, you have to make them understand how it works.
If someone asks you for a free knockup, you have to make them understand that they are asking you to essentially cut out or minimise the part of the process that has the biggest impact on the quality, effectiveness and relevance of the final result.
Most of my clients don't ask for a mockup and to my surprise a long time client this time asked for one because he's asking cost estimates to other suppliers and also the respective mockups...
I don't really like the idea, because there is a chance of not getting the job, but even so, my advice is to make a mockup good enough for the preview but not good enough for the final piece, in other words, something that points them into where you're leaning in terms of design.
Also lawndartcatcher's suggestion of a legal document is very good!
In other cases, unless it is worth it or you really need it, I don't "accept" making free mockups and the clients most of the time understand that.
It is actually your choice. I remember when i first starts make carrier i used to make lots of free mockups. But as you go further in the road, you make some reputation then you won't have to.
But when you provide mockup it is always possible that people will run away with your work! I have seen lots of these kinds of projects in freelancing sites where clients wants to make psd from jpgs. :( That's why my recommendation is strong portfolio. Doing a demo video is also a good idea.
If you need the client and are low on work, I don't see a problem with doing a mock-up to sell the client on giving you the job.
On the other hand, if you have plenty of work, it would seem unwise to do something like that. In addition to this the client who is asking for a free mock-up might be quite a bit of trouble and waisted time down the road. I have dealt with clients like this who have turned out to be very indecisive and cheap.