99 times out of 100 you'll get a "yes" when asking a total stranger if you could take her portrait. If you do it the right way. The interesting thing is, most of the times, they will thank YOU! In many parts of the world, it will be seen as an honor, a privilege.
Remember these five mistakes and you'll almost never get a "no" again!
They don't know what you are going to use it for
First of all, people in western countries usually worries a lot more about where those images are ending up. You don't want to find yourself in a hemorrhoid commercial or receive a link from a co-worker to your portrait at some obscure porn-site. We've heard so much about those things that we are scared shitless. But it takes very little to comfort someone, she just needs to hear a decent reason why you are walking the streets taking picture. And it doesn't have to be a James Bond under-cover story. "Im a photographer" is usually good enough! Don't make anything up, just explain why you are doing what you do.
They don't want to be treated like a piece of meat
There is a difference between taking someones portrait and TAKING someones portrait. Read that sentence again. The person you are photographing is actually giving you something and you should return that favor. Don't just snap a photo and walk away while your victim is trying to find out what just happened. That is as pleasant as being in a apartment when it is being raided by SWAT equipped with flash grenades. Don't go out there to take photographs, take your time and be genuinely interested in the people you meet. Make friends.
It is a religious issue
This is the absolute hardest obstacle to get over and usually the one that I tend to respect without any questions. From natural tribes that truly believe that the small black box will capture their soul if you click it their way to some certain groups of rastafaris that are not allowed to appear in photographs. Make som research about the place you are going to, that will help you a lot!
You act like a creep
The worst thing you can do when you see an interesting face is to lurk around, trying to convince yourself that you CAN walk up to her and ask to take her portrait. Think about it. What would you believe if someone stuck around a little to long, looking away every time you look his way? Whenever you hear your brain say "That's an interesting face!", just go for it. Don't wait for your head to start making up excuses why you shouldn't ask. People will turn you down much less often than if you play "dirty old man" for a couple of minutes before.
They are shocked
When someone dash towards you, shoving a camera in you face and shouts "Picture, picture?!?", you'll usually say no. Calm down, approach with a smile, make some small-talk and chill out. Find out if the personality matches his interesting face. Try to build rapport with your model before you take the pictures.