Did you look around for confederates? Move sideways, glance in the direction you just moved from. Tells them you ain't to be effed with.
On the "approach" phase, a direct look at them with a "No" shake of your head is effective.
Bingo!
Criminals will usually attempt to approach you with a ruse, or opening phrase.
The classic approach is- Criminal A approaches with the ruse, while criminal B catches you off guard from behind while you're distracted/engaged in dialogue with Criminal A.
The best response on Good Guy's part is to make body language clear you are not interested with one or two palms up in the international "no", while circling to 3 or 9 o'clock in order to see the shift the area that was behind you, into your peripheral. Use of peripheral vision is important, because if you turn your head to glance over your shoulder, it opens a window of opportunity for Criminal A to false crack you, if he has moved into range.
Getting further in depth into ruses, the following are common-
"Hey man, you dropped your money"
"Hey man, is this yours..."
"Do you have the time"
"Do you have a light"
"Can I have some money for..."
"I need some help for..."
"How do I find..."
Another area of study is "pre-assault cues". Pre-assault cues would be the movement, gesture or "tell" an aggressor would show before they actually attack.
Watch some YouTube videos of assaults and you'll commonly see movements like-
Weight shift (In order to move weight before a strike)
Glances to side or over shoulder (nervous or actually looking for witnesses)
Grooming cues like touching the face, chin, ear, or hair
All of the above would fall into Management of Unknown Contacts, or how we deal with strangers who approach us.
I learned most of the above info at a Management of Unknown Contacts module during a course on the mainland with an instructor named SouthNarc. He has a few DVDs, but his Management of Unknown Contacts video has a lot invaluable information on criminal ruses, and ways to avoid engaging in dialogue.