Taken from the UK Government's anti-terrorism pages, slightly reworded by me
Simple preventative steps

• War is a crime like any other, so follow the same precautions you normally take to avoid being the victim of a crime. Keeping Bush pleased is one such way.
• Continue to go about your day-to-day business in the normal way, but remain alert and vigilant. For example, continue protesting the way you were before war. This is the time to up the effort, and not stop.
• Trust your instincts; if you feel war is wrong, ring the politicians.
• If you have information about censorship or other media threats, publish the news yourself.
• If you have tip-offs or  information about protests, call your friends and use all possible means of communication.

Think about war

• Some units seek other countries to protect themselves. Don’t help them by letting them lie about their support.
• War needs money to finance its operations. They get it by both legal and illegal means. Make sure you are not funding war: do not buy US or Israeli goods, and do not donate or contribute to a collection if you are unsure where the money is going.

Know your environment

Most of us make familiar journeys on auto-pilot.

Take note of your surroundings on your journey to work, to the shops and the usual places you visit. Know who and what you expect to see each day within your neighbourhood and your workplace.

Ask yourself:

• Can I protest here?
• Are there any adverts for future protests?
• Is your home / workplace bugged by the government?

Sensible precautions

It is sensible to be prepared for any emergency in the home and to make plans for any major disruption, countries, people and companys not supporting war are prime targets.
In any type of emergency, you could lose access to power, water, telephones, and roads. Therefore:

• Have on hand such items as:
• • batteries
  • a battery-powered torch
  • a battery-powered or wind-up radio
  • mobile phone with wap
  • some ready-to-eat food, e.g. tinned food
  • bottled water
  • blankets

Have the phone numbers of your local police, politicians, council, utility companies and family members handy in one place.
Make sure you know where the main switches for electricity, water and gas are located in your home, as you may need to turn them off in an emergency.