First, take a few moments and look behind the urge. What are you feeling? Are you angry? Frustrated? Restless? Sad? Craving the feeling of SI? Depersonalized and unreal or numb? Unfocused?

Next, match the activity to the feeling. A few examples:

Try something physical and violent, something not directed at a living thing:
Slash an empty plastic soda bottle or a piece of heavy cardboard or an old shirt or sock.
Make a soft cloth doll to represent the things you are angry at. Cut and tear it instead of yourself.
Flatten aluminum cans for recycling, seeing how fast you can go.
Hit a punching bag.
Use a pillow to hit a wall, pillow-fight style.
Rip up an old newspaper or phone book.
On a sketch or photo of yourself, mark in red ink what you want to do. Cut and tear the picture.
Make Play-Doh or Sculpey or other clay models and cut or smash them.
Throw ice into the bathtub or against a brick wall hard enough to shatter it.
Break sticks.
I've found that these things work even better if I rant at the thing I am cutting/tearing/hitting. I start out slowly, explaining why I am hurt and angry, but sometimes end up swearing and crying and yelling. It helps a lot to vent like that.
Crank up the music and dance.
Clean your room (or your whole house).
Go for a walk/jog/run.
Stomp around in heavy shoes.
Play handball or tennis.

Do something slow and soothing, like taking a hot bath with bath oil or bubbles, curling up under a comforter with hot cocoa and a good book, babying yourself somehow. Do whatever makes you feel taken care of and comforted. Light sweet-smelling incense. Listen to soothing music. Smooth nice body lotion into the parts or yourself you want to hurt. Call a friend and just talk about things that you like. Make a tray of special treats and tuck yourself into bed with it and watch TV or read. Visit a friend.

Do something that creates a sharp physical sensation:
Squeeze ice hard (this really hurts). (Note: putting ice on a spot you want to burn gives you a strong painful sensation and leaves a red mark afterward, kind of like burning would.)
Put a finger into a frozen food (like ice cream) for a minute.
Bite into a hot pepper or chew a piece of ginger root.
Rub liniment under your nose.
Slap a tabletop hard.
Snap your wrist with a rubber band.
Take a cold bath.
Stomp your feet on the ground.
Focus on how it feels to breathe. Notice the way your chest and stomach move with each breath.
[NOTE: Some people report that being online while dissociating increases their sense of unreality; be cautious about logging on in a dissociative state until you know how it affects you.]

Do a task (a computer game like tetris or minesweeper, writing a computer program, needlework, etc) that is exacting and requires focus and concentration.
Eat a raisin mindfully. Pick it up, noticing how it feels in your hand. Look at it carefully; see the asymmetries and think about the changes the grape went through. Roll the raisin in your fingers and notice the texture; try to describe it. Bring the raisin up to your mouth, paying attention to how it feels to move your hand that way. Smell the raisin; what does it remind you of? How does a raisin smell? Notice that you're beginning to salivate, and see how that feels. Open your mouth and put the raisin in, taking time to think about how the raisin feels to your tongue. Chew slowly, noticing how the texture and even the taste of the raisin change as you chew it. Are there little seeds or stems? How is the inside different from the outside? Finally, swallow.
Choose an object in the room. Examine it carefully and then write as detailed a description of it as you can. Include everything: size, weight, texture, shape, color, possible uses, feel, etc.
Choose a random object, like a paper clip, and try to list 30 different uses for it.
Pick a subject and research it on the web.
Try some of the games and distractions at digibeet's page; she's assembled a lot of distractions.

Draw on yourself with a red felt-tip pen.
Take a small bottle of liquid red food coloring and warm it slightly by dropping it into a cup of hot water for a few minutes. Uncap the bottle and press its tip against the place you want to cut. Draw the bottle in a cutting motion while squeezing it slightly to let the food color trickle out.
Draw on the areas you want to cut using ice that you've made by dropping six or seven drops of red food color into each of the ice-cube tray wells.
Paint yourself with red tempera paint.

Get a henna tattoo kit. You put the henna on as a paste and leave it overnight; the next day you can pick it off as you would a scab and it leaves an orange-red mark behind.