Dunno, really” was the reply I got when I asked a fellow protester at the G20 demonstrations last month why he had come out on the streets. It wasn't the most auspicious start to a conversation, particularly when you're supposed to be standing shoulder-to-shoulder demonstrating for a better future while surrounded by police in riot gear.

Other platitudes followed: “I think there should be less poverty and we should look after the planet more...” These generalised moans are fine as a bit of pub philosophy but hardly clear political demands. In fact, what is so striking about recent “anti-capitalist” demonstrations is the absence of a shared purpose. Outside the Bank of England, as G20 protesters battled the police, the incoherence of our frustrations became clear: the only chant we could muster was “Whose streets? Our streets.” What most people will remember about the protests is the tragic death of a passer-by.

“I'm just here to raise awareness,” was another common response from demonstrators. Actually “raising awareness” is now a standard part of demo discourse: from the Put People First coalition to Stop the War marches, protesters gather to make the public “aware” of a cause. No one seems to think beyond awareness, or believe that convincing the public to join your cause counts for much.

If we are unable to articulate what we're demonstrating for, the act rather than the aim of protesting takes centre stage. For my generation (I am 23), being seen on, or affiliated to, a demonstration seems as important as knowing why you're there. The Make Poverty History wristband was an early example of this emphasis on form over content. The Put People First coalition's website encourages involvement through signing up for e-mail alerts, having a desktop widget and buying a T-shirt. No wonder a recent Sunday magazine did a glossy feature on protest fashion. Apparently “demo chic” is inspiring the catwalks. It is ironic that I attended the G20 protests because I didn't just want to be a spectator on the sidelines of society's problems, yet I ended up as part of an outré fashion spectacle.

The fashionable “raising awareness” mantra dodges the intellectual bullet of why anybody should bother to protest in the first place. As a political activist, I have struggled with numerous demonstrations that fail to clarify what they are for and the change they want to see. Striving to understand the world in all its complexity and working towards ideas to change it isn't academic fancifulness: it's the very thing that gives protest real bite.

Without clear political ideas you have confusion. People told me they were on the G20 demo to oppose global poverty and the negative consequences of the recession. But, ironically, almost everybody saw austerity not as a problem but as a solution. Using less energy, reining in “greed” - all were posed as the only real answers to our problems. The crowd indulged in banker-baiting and bash-the-rich slogans, effectively demonising material wealth; I argued that fighting poverty means creating more wealth. It was a tough gig.

But then politics and political arguments are tough. In January this year, a group of young frustrated people (of which I was one) set up the Modern Movement, arguing for better modern infrastructure and against the anti-flying culture so popular among climate change activists.

We were sick of the Government's equivocation over the third runway at Heathrow and made that a focus of our protests. We wanted to support a development that brings more convenience, more flights and less hanging around at airports. And yes, I value the opportunities that flight can afford my generation: who wouldn't want a cheap weekend escape to Prague?

Such self-interest may sound selfish but its aims can be beneficial - we wanted to highlight the positive gains that modern transport and the freedom to travel have given millions of ordinary people.

But these are hard arguments to win, and go against the grain of all the other protests around the Heathrow runway. Trite clichéd slogans wouldn't cut it. We realised what we needed to do to persuade people was: write articles, organise debates and think through all the issues.

I was also involved in running the Manifesto Club's Campaign against Booze Bans last summer, defending the freedom to drink in public from overzealous regulation. Experience of the campaign taught me that even civil libertarians, fighting the fair fight for formal rights, can quickly turn into illiberal reactionaries about lifestyle issues, such as the freedom to drink in the park or have a fag in the pub. Again, these are difficult and complicated issues, and protesting over them means winning some hard arguments.

Protest is not dead. There are things worth fighting for. Many of us are involved in politics not because we are pious awareness-raisers. You could well ask: what were you doing at the G20 demo? It's simple: I wanted to win my peers over to political ideas I think worth standing up and fighting for. You may disagree with them. But rather than “raise your awareness”, it's down to me to win you over.

Suzy Dean is speaking at the Battle for the Economy, on Saturday. For information about tickets, go to www.instituteofideas.com

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