Just watched a programme on BBC: http://www.bbc.co.uk/programmes/p05x9kfp on moped gang crime in London
I learned some important messages, which I think we should all be aware of:
They will go for easiest targets (obviously), including anyone at pedestrian crossing, anyone on footpath - anyone who is visibly using their phone, especially of wearing headphones (because these will be less aware of their surroundings).
They will target bags which do not seem fully secured to their owners
Always keep your phone in pocket when in public place.
Don't think that if you are in a busy public street in London they cannot target you - if anything, this makes it easier for them when they want to run away.