Portrait lenses (also called standard and medium telephoto lenses) give you a more natural view and perspective. Basically, what your eye perceives is what you will get in your shot.
These lenses give a more flattering and natural perspective. Your subject is isolated and your overall image appears more flattened, making your model pretty as a picture.
Take advantage of the really cool "telephoto effect". Telephoto lenses can appear to flatten a scene. It makes it look like all subjects are close together, when they are in fact far apart. It can have quite a dramatic effect.
Take advantage of the aperture available to you on a telephoto lens. It blurs a potentially distracting background, allowing your subject to be the star of your shot.
Even the smallest movement is magnified with a telephoto lens. This can spell a blurry disaster for your shot. The trick is to use a fast shutter speed to ditch the shutter shake. Or use a sturdy tripod. Some lenses come with an Image Stabilizer mode that instantly helps clean up your shot.