I am working on a new web site offering intense and fun stock trading
games and contests.  While I am a good software engineer, I am not
much when it comes to accounting.

I need help to model a portfolio of say $100,000.  I want to model
margin, short sales, account equity etc. etc.  I have most of the
model working but really need help from an expert in properly setting
up margin, short sales etc.

I need a complete model with how to calculate margin, how to account
for short sales.  For example I know how to do long buys and sells
without margin:
Port Value: $100,000
Buy: 1000 of Stock AAA at $10
Cash Balance: $90,000
Equity Value: $10,000
Port Value: $100,000
Stock increases: AAA is $20
Cash Balance: $90,000
Equity Value: $20,000
Port Value: $110,000

With shorting I have a problem.  If user sells short I cannot just
increase the cash balance ... because I pay interest overnight on the
cash balances.

So in this case cash balance, equity value, and port value are the
fields I track.  The model you provide should include the fields I
need to properly track short sales, margin, total equity etc.

I could maybe hack it out myself but would prefer a very real solution
based on real accounting as it is done at online brokerages ... an
approach that is commonly accepted in accounting circles.

If you are an accounting/finance whiz please respond.
If you want to help on the project please let me know!
Will pay more for the right help ...

But the cash balance from the short sale is not available to the
client.  You take a "short position" or "go short."  It *subtracts*
from the client's buying power.  It does not add.  As the market value
of the stock you shorted declines, your equity goes up.  The short
interest for open positions is usually adjusted once every x
days/weeks and not daily.  Quite often the ultimate authority for
margin arrangements is the broker dealer themself.  The SEC/NASD
restrictions are actually fairly lax as compared to what is enforced
by the B/D to manage their own risk.

Many B/D's have FAQ pages, like this one:
http://www.tcfexpress.com/OnlineBrokerage/online_brokerage_faq.jsp

You may also review the gov't publications like:
http://www.sec.gov/investor/pubs/margin.htm

PS - I bet the guy who runs www.daytradingstocks.com would be happy to
link to your site when it is ready.  Best wishes.