I've never been mugged, but I'd like to think I'm fairly street smart, and would be able to handle myself in such a situation.
However, I know in some hostile situations, I get shaky as adrenaline starts pumping. I usually find steady breathing helps deal with this, as appearing shaky will make you seem more vulnerable.
Recently, when dealing with people just being aggressive for no particular reason, best thing to do at first is ignore. If they persist, calmly confront them, making sure not to raise your voice, and try to rationalise/reason with them. This will hopefully either have the effect of getting them to sympathise with you and think about what they're doing, or make them bewildered and realise they're not going to get the reaction they want out of you, and leave.
For example, though at the time it was more of a whimsical response; Myself and nine friends, all of us being the ages of 18/17, went down to the beach for the day. Everything was great, no one was bothering us at all, and the beach was quite busy. We had just had lunch, Dominos pizza, and all the pizza boxes were still by all our stuff. Most of us went into the sea for a bit, leaving one of our friends, a girl who didn't want to swim, to look after our stuff. When we got back to our stuff, I noticed two kids, the oldest was probably only 12 years old, sat really close to us, and they were both playing in the sand. I thought it was odd that they decided to sit so close, but whatever. A minute or so later, sand was being thrown subtly at us, but I figured they were playing and hadn't noticed where the sand was going. But then bigger bits of sand were being thrown, and now I realised it was quite deliberate. I whispered to my friend next to me to ignore them and they'll get bored, as I was having a good time and didn't want to get hostile, especially with kids.
But eventually my friend next to me turned around and glared at them, to which one of them replied, 'What?', but then he proceeded to turn around and forget about it. Soon I realised my mistake in ignoring them, as they felt they had some form of dominance of us, as they started to reply to conversations my friends were having, now beginning to throw pebbles too.
It was at this point both my friend and I turned around, to which they again replied, 'What?'
We asked them why they were throwing things at us, and told them to stop. The youngest hastily defended themselves, then inquiring if we had 'proof.' I told him we don;t need proof, you were just throwing it at us.
My friend tried to reason with them, offering that we were fine with them being there, as long as they weren't dicks. 'We haven't done nothing!' was their plea.
At this point I was bored of them, and wanted to carry on enjoying my day, so I leaned closer to them, and started to ask them, 'Why do you guys do this? Why can't you just live with some goodness, and peace, y'know`?' I went on like this for a minute or so, and they didn't reply. Upon realising that they weren't going to get a rise out of us, they got up and left.
Once gone, our friend who we left with the bags told us how they said to her, 'I'll take your pizza, put it on your vag', and lick it.' Which bewildered me in two ways; one: what kind of kids say that, and two: who the hell licks pizza?
It later came to me that they were trying to creep her out, and get her to move away from the bags, as we saw them prowling around the beach, looking at people's bags, trying to knick stuff left by beach-goers.
And so that is the tale of how I, with the power of verbal reasoning, fended off two hostile children.
... What also confused me is why two kids of their age felt confident enough to antagonise a group of 18 years olds. Maybe they were used to interacting with them, as they probably lived at this particular seaside town.