Running a hard shoe heel down an attackers' shin can take their mind off things for a while, as can standing on their foot in a stiletto heel and pushing with all your weight.
If you are wearing soft trainers, the scraping down a shin isn't as effective, but you can still make a pretty good impression on their instep if you tread down hard enough. If you're wearing flipflops or those flappy sandals, kick them off because it's bloody hard to run whilst wearing them. Aim to get the attacker off balance whilst remaining upright yourself.
If someone puts their hand over your mouth from behind, stick your tongue out and lick it, get it nice and slobbery... it's amazing how many people instinctively pull their hand away and if that doesn't work, the saliva may act as a lubricant so you can twist out of it and bite a finger.
A bunch of keys held in the hand can make a pretty good knuckle duster - find a really big, robust metal keyring or use a climbing carabinner that your fingers can fit into.
Don't carry a blade. Chances are it's a) illegal in your country/state, b) it'll be in the bottom of a purse, case or pocket where you can't get to it quickly and c) an attacker will use it against you. If you really feel the need to carry something and want to remain legal, then a steel nail file with plastic handle can be used to pretty good effect - the point is usually sharp enough to break skin and the rough file surface will collect enough skin for DNA analysis. It's also a fairly innocent piece of daily junk that anyone might have, so if you do get stopped by the police, you're in the clear.
Walk confidently, keep to the road edge of the pavement/sidewalk and try to avoid using your mobile phone (cell) in public areas. If you are going anywhere alone to meet with someone you barely know (like a blind date or to buy a vehicle/house), have a friend primed with the name and address of your destination and an ETA back home. Text or leave a voicemail when you get to the destination and again when you are home safe.
Don't tie your scarf in one of those loop knots, or wrap it completely around your neck - you'll just create a nice noose for anyone to immobilise you with.
If you use a shoulder bag, wear it diagonally across your body rather than looped over one shoulder. This makes it harder for a bag snatcher to grab it so they'll be less inclined to try. Also, you won't waste energy hoisting it up or risk dropping it and tripping in the strap if you have to run. In bad weather, wear your bag inside your overcoat as this will make it harder to spot and anyone grabbing you just gets your coat.
Never ever EVER carry your purse or wallet in a back pack. Make sure the zips are secure and preferably are the sort that have two zippers on one opening - use both sides of the zip, have them meet at the top and either split ring them together or use a small padlock. If the bag is the sort that has a flap that goes over the main pockets, make sure it's velcroed down and the zippers can't be seen. If the bag has two straps then use both straps, don't hang a back pack over a shoulder by one strap, it's too vulnerable. Don't put your mobile phone in the little pockets they put on the front for mobile phones.... it's like writing 'BACON' on the side of a pig.
Have your banks contact numbers written somewhere other than on the back of your credit cards - if they get stolen and you're not near a phone book, how are you going to call and get them cancelled?