One flash rule to remember is that the larger the light source, the softer the light. The smaller the light source, the harder the light. This means that the larger soft light will have softer edges in the shadows and the smaller hard light will have a more defined edge at the shadows. This technique also works very well with ceilings.
One very easy way to turn the small light from your shoe mounted flash into a large soft light source is to bounce your flash light off of a nearby wall. You do this by having the person you are photographing move near a wall or large solid surface, then you rotate your shoe mounted flash so that it is facing the opposite wall and not directly at your subject.
When you fire the flash it is going to bounce off of the wall that it is aimed at and turn into a much larger light source, thus creating a softer edge for your shadows.
One thing to keep in mind when using this technique is the color of the wall. The color of the wall is going to influence the color of the light hitting the subject. By this I mean that if you have a white wall to bounce your flash off of it is not going to be that big of a deal, but if the wall is, let's say red, then that will tint your light with red.
There are a few different way to counteract this light tinting problem. The two easiest way is by either changing the white balance of the flash, or by changing the light from the flash with a colored gel. The exact color and white balance combinations will depend on the color of the area, the wall, and also by what color you want the light to be.
Depending of the color of the surface I will usually start with the white balance for color correction. When it is warm or cool colors changing the white balance is the easiest. I suggest you correct the white balance in camera during the shoot, but you can change it in post as well.