Mace, schmace. Burning of the eyes is nothing compared to a 125 dB alarm going off in your ear and the AlarmWear line of apparel and accessories can accomplish just that. AlarmWear manufactures backpacks, jackets, fanny packs (don't giggle) and vests that include a 125 dB alarm (think shotgun or air raid siren) that will sound with the pull of a ring.
There are quite a few techier ways to protect yourself on the streets, but at least you can't screw up this device. I'd be worried about getting the loop snagged on something or accidentally pulling it and all hell would break loose. The AlarmWear line ranges from $40 to $60 and is even available in kids sizes.