Like any lighting, this is not a simple one-size-fits-all type of deal. You have to think it through. Light should very much feel organic in the image and should have a reason for being there (i.e., "motivated light"). Nothing is more jarring than a light source in an image that your brain tells you just shouldn't be there. So think what light sources in the scene should be that aren't or aren't registering strongly enough (overheads, window light, etc.) and then try to use the speedlight to imitate that. Think in terms of the direction and quality of the light, and learn to use the ambient to your advantage. Don't expect to light an entire large space with just a speedlight without also using a high ISO setting and a wider aperture.

It's easier to control the direction and quality of the light if you can get the flash off the camera using remote radio triggers and onto stands with modifiers. But you can soften the light and choose some different directions by bouncing the light--that is, aiming the head of the speedlight towards the area you think the light should come from, and use the bounce/reflection surface as your light source. To avoid having any direct spill from the head, you might also want to flag it off.

Scott Hargis, in his Lynda.com course on lighting for real estate, often puts a speedlight on its small plastic screw-on stand, and balances it on the top of a door, and rotates the head to face into a nearby corner, so he can get soft bounced light for the rest of the room.

See also:

• Scott Hargis's lynda.com course on Real Estate Photography, and blog.
• The photographyforrealestate.net site's lighting section
• Neil van Neikerk's Tangents site for on-camera flash techniques
• David Hobby's Strobist site for off-camera flash techniques