The answers would appear to be investment. Not money - but investment of time, investment of guidance and investment of stong local leadership.
It's no use simply blaming the parental discipline, gang culture, grime (that's music, rather than living conditions), schools or social services
I myself am investigating how to be taken on as a mentor (via the Big Brother Big Sister organisation), but forthcoming cuts to the LDA budget make the likelihood of the scheme being rolled out across London seem fairly unlikely.
At the other end of the issue (fincance, rather than community action), cuts to the Metropolitan Police budgets and the abolition of the Met Police Authority mean that delivery of Boris Johnson's Mayoral objectives have lost the good will of the Police and this is vital for any Mayor.
That's coupled with cuts to youth crime diversion and employment projects. Schemes such as the LEAP project in West London are to lose their funding, and the Code 7 youth project, and the Starlight Music academy in Brixton are being sued by Boris and evicted from their premises despite providing an excellent support service to young people.
So, all in all - no easy answers. We COULD say, "Right...anyone convicted of carrying a knife in public will automatically face a 10 year prison sentence" - but that's addressing the problem 'from the wrong end'. The issue needs to be tackled at source, and I doubt that there's the political will to this. It will take money, and it will take the complete re-education, and re-direction of an entire generation of kids.