Try a product called Feliway (you can find it online or at petsmart).
It's kinda expensive, but a lot of people on these groups swear by it.
It gives off a smell that calms down cats. Other than that, I would
say just be very patient and give the cat time to get to know and
learn to trust you.

On Fri, 22 Aug 2003 08:37:06 -0400, "Doug Stackhouse"
<doug@stackhousesolutions.com>  tossed a golden coin into a well and
made a fervent wish:

Glyph

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http://glyphworks.crimsonblog.com
http://mundanechaos.keenspace.com

Has the cat been taken to a vet to check for
medical conditions?

And are you sure it isn't just shedding as it's
summer? Daily brushing cuts down on the amount
of hair floating around the house.

It can take cats several months to settle into
a new home. Especially when they have been
homeless, or shuffled around earlier in life.
Calls for patience, and gentle treatment.

A number of Pet Smarts run as centers for humane societies or other
shelters, and if that goes for the Pet Smart from which your new addition
came, find out which shelter, and see if they have any medical history on
the cat. Pet Smart itself might have these papers, or papers of their own.
In any case, ANY new addition to your home should be brought to a vet as
soon as possible to verify for spay or neuter (if it isn't done before you
take the animal home), and to get vaccinations and a general check up. Even
if you have a small apartment, introducing a cat to a new environment can be
VERY stressful (unbelievably so for some cats, while others don't really
seem to mind and just take charge of the whole place and all the residents).
If this cat was simply let loose in the house, you may want to set up a
bedroom or even the bathroom with litterbox, bed, food and water, and let
the cat get used to that one space for a few days or however long it takes
for kitty to come out for some love from his new people. Then, expand his
territory a little every couple of days after that. Just remember to take
some time to try out treats on him, find out what he really likes, and use
that as encouragement. If he's in a bedroom or a space large enough, stretch
out and read for a little while, down on the floor on his level.
A LOT of cats will shed like mad when they're stressed, and Feliway might do
a lot to reduce that stress, as will a lot of patience and love on your
part, and especially your girlfriend's part.