However tempting it might be in the movie and video business, don't work for nothing!
There's no way to start a piece like this without some sort of disclaimer, so I'll be straightforward about it: the following is an opinion piece and is intended to apply to the UK, although I expect it'll be fairly familiar to most of the world's film and TV workers who aren't constantly employed on multimillion-budgeted feature films and commercials for the world's top five companies. Which is, after all, most of the world's film and TV workers.
It's very important to be clear exactly what we're talking about here. Situations where established professionals and collaborators of long standing agree to do favours and help one another out on the occasional project are hard to object to, and the concept of a formally agreed and planned apprenticeship as a learning relationship between experienced and inexperienced people is ancient. The problem is the whole concept of interns who are compensated neither by money or teaching, whose employment is almost entirely unregulated, and who are starting to make headlines in all industries, let alone filmmaking. It's a problem that's infected more or less any industry since the economy went south, but has been growing in moviemaking for a decade at least and is now more common than ever.
Even once we've excepted the old-pal charter of working with existing, paying clients, it's still not a simple situation. There must reasonably be a moral difference between funded productions that choose not to pay people, and unfunded productions who can barely afford their camera equipment and couldn't choose to pay people if they wanted to. Still, productions which absolutely could not afford to pay even a token fee – possibly the national minimum wage, as if that's easy to enforce on a schedule as flexible as that of a film – as a mark of respect are probably quite rare, and almost all producers (especially producer-directors) should be willing to accept that filmmaking is a collaborative art. Since many of the collaborators do not receive significant artistic credit for the results, they should reasonably receive financial compensation instead.
But whatever the motivation, it seems that there will continue to be productions which offer unpaid work in the hope that people will view it as worthwhile experience. The real question, therefore, is how reasonable that view is, and how worthwhile the experience might be, whether it's about observing and learning, or making the tea and paying your own travel expenses. I think that most of the unpaid positions that exist on short films and low budget music videos actually are, quite often, expected to be skilled, regardless of the ability of beginners to fill the role. Producers generally only try to get away with the human-teasmade jobs on higher-end stuff, where it's perhaps less excusable, given the budgets involved, but might actually do more good in the least-tangible senses of contacts and visibility. So, a good first principle might be to find out specifically about the proposed work, in writing, and gauge the response against the quality of the production, overlooking, of course, the tendency of producers to present all projects as Oscar contenders. At some point it's about trust. If something smells like a prospects-free two-person shoot behind the bike sheds, it probably is, and it's therefore possibly worth giving a miss.
The darker side of working for free, at least in an oversubscribed industry, is that in extremis there will always be someone willing to do for free a job that would otherwise have been done by someone for money. Choosing to work for no money for one's own sake is one thing; depriving someone else of gainful employment is quite another. Whether any particular job represents a risk of this can be difficult to establish, especially on that hard-to-determine middle range when it isn't obvious how much money is really available for crewing. Asking about which people and equipment are already involved is one approach, since the presence of well-known or very experienced people, or very high end equipment, indicates that there is probably money that they'd prefer not to give you.