If you want to daydream, don't do it on a city street.

Men who attack women -- whether to rob or rape them -- are likely to pick someone who looks vulnerable, who they believe will not fight back or make a scene, who is not alert to her surroundings, self-defense experts say.

Although circumstances vary, and at times it is not safe to resist an attacker, local self-defense instructors and police generally advise women to react immediately to a threat.

"Do not be afraid to make a scene," said Chris Nichols, a self-defense instructor for the D.C. Rape Crisis Center. "This is the biggest thing with women. They are taught not to hurt, and a lot of women have trouble fighting back."

"The most important thing women can do is to think," said Detective Robert Catlett of the D.C. police department's sex offense branch. "If she's thinking, she might see that the gun he's carrying is really a water pistol or that if he puts his knife down . . . she might be able to run."

Fighting back can mean a range of things, depending on the situation. If someone is following a woman, she can walk across the street or in the middle of the street, or join a group of other people. If she is in a place that is isolated, she can scream if she feels in danger.

"Trust your instincts," Nichols said. "If you feel threatened and unsafe, you probably are threatened and unsafe."

Police and others said assailants often pick women they believe look vulnerable. Women should carry themselves with confidence, walk briskly and look like they know where they are going, they said.

"If you act timid, people will pick on you," Catlett said.

Experts said women should keep their hands free, turn the flaps of their purses so that they face inward, and have car keys ready before getting close to the car.

Some women wear tennis shoes so they can run easily. Some avoid tight-fitting clothes for the same reason.

Nichols discourages women from carrying weapons of any kind because they can be turned against them. Also, trying to use a weapon may prevent a woman from reacting to the threat by running away from it. Mace is illegal in the District, but it is legal in Montgomery and Prince George's counties and in Northern Virginia, except on school grounds.

If a woman is trapped by an attacker in an isolated area, she must assess options and "do what you have to do to protect your life," Nichols said.

Carol Middleton, who heads the Self-Defense Karate Association at All Souls Unitarian Church in the District, said that if an assailant turns violent, the best idea may be to try to talk him out of the attack. If it's just the purse he's after, "be cooperative; don't call him funny names," she said.

If that does not work, "use your voice, make loud noises and think about the targets" -- the eyes, nose, groin, knees.

Middleton said that escaping date or acquaintance rape, which accounts for at least one-half of the rapes committed, is often a question of mustering the psychological will to forcefully refuse someone who the woman thought was a friend.

"If you're feeling uncomfortable, he's not honoring your boundaries," she said.

The newest approach in self-defense training locally is called Model Mugging, in which students are taught to react aggressively to assailants and to fight them on the ground with their legs. Students practice counterattacks on male instructors -- the "model muggers" -- who wear head-to-toe suits of padding that weigh close to 40 pounds.

The Washington chapter of Model Mugging began in March and is taught by Middleton. Information on other local self-defense classes can be obtained through area sexual assault hot lines.