I shoot interiors commercially, mainly for architects and interior designers. I use portable flash if necessary but prefer to work with the light the room offers, whether it be downlighters, wall lights, ceiling lights or table lights, whatever there is try and switch it on, a lot of estate agents shots are dark and don't look welcoming.
Use a tripod, and a low ISO, long shutter speeds will help with the darkness. If you want to do a proper job of it take a custom white balance off a grey card, it will help avoid the odd colour casts you see on some EA shots.
One of the main things is to find a good angle, lots shoot square on and the room looks dull. Shoot at an angle, possibly from the next room and through the doorway and you might find a much better view.
Finally don't forget to dress the room, clear away dishcloths, mugs left out on worktops etc.
Obviously there is more to shooting interiors than that but it will get you in the right direction. Also my clients needs for full page glossy adverts may well be different to a few snaps to bang on rightmove, it all depends on your market and how much time / expense you can run to. If you're selling high end properties why not look at how much a local pro will charge to take the shots for you? You might find the improved photos mean you can market the property at a higher price and your increased commission covers the cost of the photography.