How do I prevent phone theft?

I'm sure I may have some sarcastic answers to this but this morning on my way to work, I briefly had my phone out of my bag, and a guy on a bike tried to grab it. I've had my phone stolen twice before like this, so I'm pretty careful with not taking it out in the street but surely there must be some sorts of things one can use to prevent or combat this crime (other than the obvious), it's so prevalent on London streets. Does anyone have any suggestions/helpful advice (please be kind)?