Normally, for a topic that has been so thoroughly discussed and written about I would just find the best resources and add a few of my personal tips. However, the more I read the more I felt I had to write my own piece.
1. Intent - What is the aim of the photo? Is it a travel portrait? Is it a casual street portrait of friends or family? Is it a street portrait of a stranger? Having a clear idea of why you are taking the shot narrows down how you should prepare and compose the photo. E.g. for a travel portrait, you normally want to include more background to give context. Whereas sometimes the person you are photographing may have some unique facial features which you want to shoot closeup to bring more attention to the details. How much background you want to include in the shot will affect what type of lens you use. e.g. a wide angle such as a 24mm or 35mm to include more background, and a telephoto lens such as a 85mm for close up shots.
Shot taken with a Nikkor 85mm f2.5 1/250 ISO100
2. Lens - Having the right tools for the job is crucial to achieving the results you want. Of course there are exceptions but as a beginner it is best to learn from the basics. For portraits I definitely recommend shooting with larger apertures such as f1.4 - f2.8. The shallow depth of field helps isolate the subject from the background. When you want to show more details in the background, then you will want to use a smaller aperture such as f5-f9. The most common portrait lens photographers use is the 85mm as this allows you to shoot from a reasonable distance away. For travel street portraiture, I generally carry a Sigma 24mm or 35mm f1.4.
Shot taken with a Nikkor 105mm f3.0 1/250 ISO800
3. Subject's pose - I normally prefer to shoot street portraiture while the subject is not aware so I can accurately portray them in their most natural state. The other option is to engage your subject first, get to know them a little bit more and try to take a shot of them which best portrays their story. With the latter, you will sometimes need to instruct them on how to pose for the photo as some people can be very stiff in front of a camera. How to interact and position your subject is a skill that will take some time to master as you will also have to consider the surroundings as well as the angle of light on your subject. I would suggest that you start from shooting subjects that are unaware first, practicing your timing and composition of finding that right moment to take the photo. Once you get a better handle of that, I would suggest referencing fashion magazines and other photographic publications and try to mimic them.
Shot taken with a Nikkor 85mm f2.0 1/100 ISO500
4. Background - I have mentioned this in a previous post but I can not stress enough the importance of paying attention to the background of the photo. A good photo consists of a coherent subject AND background, avoid poles behind subjects, bright colors or lights that would distract the viewer from the subject. Using larger such as f1.4-f2.8 apertures makes this some what easier as everything behind the subject becomes a blur. If the lens you have does not have such large apertures, you will need to be extra aware of finding clean backgrounds for your subject. There are many times where I come across an ideal background and just wait until someone interesting walks past.
Shot taken with a Nikkor 70-200mm f2.8 1/100 ISO100
5. Light - Photography is all about light, for the most ideal light go take photos either in the early morning or towards sunset. Generally speaking, the most flattering light comes from an angle diagonally above the subject. Mid day is not a great time to shoot as the sun is generally way to harsh and creates very contrasty photos with unflattering shadows. Having beautiful natural light is 50% of a great shot already so try to shoot during those hours. I always avoid using flash when possible as I feel the light looks very unnatural on the subjects.
Shot taken with a Nikkor 85mm f1.8 1/2500 ISO100
6. Focus - As previously mentioned, street portraits are often shot with larger aperture settings. In order to get razor sharp shots, you need to make sure that the focus is on the eyes and not the nose. When the aperture is wide open, the depth of field is so shallow that if you focus on the nose, the eyes will be slightly blurry already. One way to overcome this is to use single point focus instead of autofocus.
There you have it, my six tips for taking awesome street portraiture! Try to keep these in mind the next time you shoot.