If your flash mode is on Automatic, switch it off. Flash can look harsh and unnatural especially for indoor portraits. So what are the tricks?
First, push the ISO up – usually ISO 800 to 1600. If you are using your smartphone, go into the PRO mode. Use the widest aperture possible – this way more light will reach the sensor, and you will have a nice blurred background. Using a tripod or an I.S. (Image Stabilization) lens is also a great way to avoid blur.
If you absolutely must use flash, then use a flash with a head you can rotate, and point the light to the ceiling on an angle.