Removing all mundane traces of the person is important. Take down their clippings, throw out the phone book with their notes scribbled in it, sweep the place thoroughly. Wipe down desktops and telephones and other grubby bits of furniture, maybe with a clean scent like pine in the wash water.
Sprinkling with salt water is a good idea--as far as I'm concerned any kind of salt will work. Incense or perfume can also work, but are more likely to annoy co-workers. For me, even better than sprinkling is walking the boundaries of the space and dabbing water by hand onto walls, doors, windows, etc.
Then put up something of your own to claim the place. It doesn't have to be explicitly pagan--photographs, perhaps a calendar with nature scenes, a cartoon clipping, whatever will help mark it as yours. Perhaps ask for a blessing on the room, if you are comfortable doing so, or make a dedication of it to your working purposes.
Suggested reading (pdf e-books):
Rodolfo Amadeo Lanciani - Pagan And Christian RomeMaroth Miklos - Picatrix First Chapter In Hungarian
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