how to ask people to pose in photography?

Don't ask randoms for a start. Ask people you know, like friends and family. Remind them that it's for photography reasons only and remind them that you are experimenting and just need a couple of minutes to take some photos. If they still persist, just ask for one photo and remind them that your not looking for a model for the photo, you just want to experiment with photography,

Ask people who look vain
Don't ask people with too many friends around because of embarrassment, So 2-3 ppl
But sometimes not alone because that person might need some sort of witness and support with them
Compliment them
Give them a straight forward answer
Give them a link to a Fickr page where you can upload it for them to see cos ppl always want to see the picture (use some sort of well known website so that you don't seem dodgy)
Make a business card
Don't touch them! (as in pats on the shoulder or grabbing arms for attention)

First, you have to be confident. If you're timid, they think you're up to something. If people tell me, "I don't take good pictures," I tell them "You've never had a photographer as good as I am before." Give them a reason to cooperate. Say, "I'm a photography student, and I need some street -pictures (couples, etc.) Or,"You have a very interesting look (eyes, clothes, etc). May I take your picture for my magazine assignment?" And then direct them. Tell them where to stand, which way to look, etc. This means you've thought it out beforehand. Keep jabbering while you're shooting.

Working with models and people you know, you should just get physical and put them into the position you want if they refuse (within limit obv). Particularly with models, it's their job.

But with street photograph that's not the point, you're meant to capture the nature of society in public. Use a fast telephoto prime to pick out subjects from a distance, try a 100mm f2 or a 135mm f2 or 2.8.

It's hard to ask people to post in your photograph especially those who doesn't know you. One hint on that is that you pay them to pose for your photograph or you can ask your family and friends to do so for free. I am also a hobbyist on that craft but i love to take candid photos like i was taking shots for photo journalism. It is much better since you don't have to ask them to pose. You can shoot from a distance using your zoom lens. You could also try that.