A friend of mine, a cop, has suggested a fake wallet. You throw it on the ground, and as the guy picks it up you kick his face in (Art also suggested wearing shoes with metal cleats when you do this, but realized that that wasn't always possible).

My brother took "shooting training" from the Illinois State Police (his employer, although he's not a sworn officer). At the end of the two weeks he qualified to carry a concealed weapon when he was on duty. He sometimes, as a result, packs a .38, a "Chief's Special" that holds five rounds. He feels that he doesn't need more than that because, as he said, "If I need even one round I've made a mistake and probably shouldn't be there."

Carry a small, powerful flashlight, like one of the Maglights that take 2 AA (L6) batteries. You have light, which can blind an assailant, and a heavy aluminium case which can be used as a striking weapon (inside your fist, and hammer with the projecting ends).

I've never found the old "strike with the elbows into the guy's ribs" to be very effective. That's mostly because you can't usually get enough force going to make it work well if you're tightly held from behind and especially if you're held over your arms. Smash the instep instead -- and if you are foolish enough to be wearing high heels, place one of them on the instep and shift your full weight onto that foot.

If you can turn slightly, raise one foot and violently scrape the outside edge of your foot down along the bad guy's leg, from above the assailant's knees and bounce when you hit the instep. That will scrape his patella off AND break his instep.

If you can attack the testicles, use you hand through his clothing. Grab 'em, and yank them hard with a twisting motion. And don't worry about missing -- a man will almost always break his grib on you if reach violently for that part of his anatomy. Moreover, it's harder to block a hand than a knee. (If you do kick someone there, and it's not recommended that you try -- man or woman -- catch them on the end of your shin bone. You can literally raise them off the ground and break their pelvis. Regardless of sex they're gonna hurt.)