Some tips:
1. Try to avoid firing directly at reflective surfaces, e.g. spectacles, people standing in front of windows.
2. If you can, bounce the flash off a plain, light ceiling or wall (watch for light fixtures on the ceiling which can cast reflections).
3. If direct, putting the flash above the camera makes the shadows more natural (downwards).
4. If people are too near a wall, the shadows they cast will be very sharp and possibly distracting or ugly. You could shoot on shutter priority and lower your shutter speed to maybe 30? It will require less flash, and shadows will be less obvious.
5. Know your flash strength (guide no.). Areas beyond flash range will get much darker with distance.
6. Due to the above, if people are too far from the background, they may appear a bit overexposed and the background may go black. You can compensate a little by lowering the shutter speed to 30 perhaps (depending on your lens & situation) and letting the camera balance the flash output. This is one of the more useful tips I learnt. The slower speed lets more background light register.
7. Outdoors, fill flash can be used to minimise shadows. If afraid of overexposure, some cameras can be set to have e.g. -1 flash compensation (different from normal exposure compensation).
I don't like using flash either, but these are the things I find more useful to remember.
