Off the wall!
Direct flash is extremely harsh and often casts unwanted shadows. Try bouncing the flash off of a reflector next to the model for a much softer light.
The following results used an SB-600 external flash unit attached to a Nikon D80 angled towards and bounced off a wall.
For different lighting effects, try adjusting:
Experiment with different angles, distances, and placements! Try it out and see for yourself.
Again, you can substitute a reflector with a wall. The external flash can be attached to your camera or separated and attached to a stand for more flexibility in angle, distance, and placements.
Off the ceiling!
Try bouncing the flash off of the ceiling above the model instead for a much softer and natural looking light as achieved in the results on the right.
This technique works great with an external flash unit or a pop-up flash. Just use a note card to bounce the pop-up flash lighting towards the ceiling. Refer to a search engine for more ways to bounce light with a pop-up flash.
Of course bounce lighting is not limited to portraiture. It's great for any indoor shoot where walls are plentiful or even outdoor shoots where reflectors are available.