Bouncing flash is the way to go. Depending on situation and the surfaces we have around us, it may take a few practice shots to get it right. Given only one chance I'd choose the most sure way to bounce, but when time allows to search for it, my preference is this:
I turn the flash head to point up towards ceiling over my left shoulder. Yes, that's actually behind me then. Let's say 60 degrees up and horizontally 120-160 degrees away from my subject.
If it then appears the flash is not powerful enough to light my target properly, I'd try bouncing from upper portion of the wall on my left hand side.
If even that is not good enough, then the same directions onto your right hand side.
Last chance for indoors natural surfaces is the ceiling directly above me, or even a bit in front of me, if that is needed to get enough of light down on my subject. This is often not very satisfying way to bounce, but still in front of the item number 5 on my list.
If all above fails to lighten your subject properly, I'd try an impromptu bounce surface like a white letter cover, electricity bill, business card even, and again bounce from left hand side, i.e. turn the flash head directly to your left and hold the impromptu surface in 45 degrees angle in front of flash head.
Using a diffuser, if big enough, can be even a desirable kind of lighting, giving a certain soft but direct light on your subject. I happen to dislike it, so in my case using diffuser is the last item on my list. However it may be good to give it a try, you might like the result more than I do.
When there is a good deal of natural (or artificial) light present in the room, things become a bit more locational, I mean, more depending on the light source, the subject, the surroundings. The natural light could be given preference and flash would be a gentle fill light bounced from the opposite direction to the natural light. Or the natural light would be the fill light (downplayed with exposure settings) and flash powered up to become the main light. If there's plenty of natural light, then it might in some cases even be a place for direct frontal flashing with low power, or with a large diffuser in front of flash. With a lot of ambient light it also becomes a matter of having good or at least acceptable white balance in the lighting. If unsure, set exposure to diminish the ambient light and use the power of your flash freely.