Up Bys!
Originated as a term used to describe pulling a wheely on ones bicycle but has since evolved into a commonly heard piece of street banter.
On seeing your friend on the other side of the street, or perhaps whilst passing in a car with the window rolled down as a friendly rousing gesture shout "Uuuuuuuuup bys!" with raising pitch on the 'up' and even higher pitch for the secondary 'bys'.