Sometimes during slow periods I have nothing to do at work (I'm a
legal assistant at a law firm) and every once in awhile I guess my
boss feels it's necessary to walk past my desk to see what I'm doing. 
Do you have any suggestions on how I can "look" busy at work (work on
fake documents, etc.)?

Perhaps this will be of interest to you:
http://answers.google.com/answers/threadview?id=573860

karib
access www.lastminute.com
Register (it will not cost you anything)
then click "Bossiswatching"

Keep that session open and if your boss is coming by just click to it.

If you really get bored you can add comments to Google answers till your
heart is content   

(but don't stiff a reseacher out of his/her fee. Then the GAR's get
really mad! :-))

regards

DPRK007

Awesome!!  Thank you!

Aren't you required by the firm to enter time records.  Surely
(assuming you aren't lying), your boss can tell from reviewing these.

I'm an attorney and when I was an assistant at a firm while in school,
sometimes the same thing would happen.  I would always have a research
window open or a motion open while I was surfing and when the bossmen
would come in, I'd just open that and say I was proofing it.

Same scenario as karib here. I use most of the 'down time' to further
my education (and that is encouraged by our firm), we have a great
library at work we use to that end. However everybody knows you can't
do one thing all the time you need a break, and so, we can do anything
we want.

The boss here loves to play chess, so I often find him on the net
doing that. Not a good idea to beat him at chess, worse than beating
your boss at golf I tell ya....  ;-)