I am a CPA as well as a professional poker player and daytrader. What "opt789" said was right.
My tax advice is only for daytraders and not for investors.
1. It's too late to elect mark to market for your 2009 tax return. You had to make that election by April 15, 2009.
2. The main benefit of mark to market is that you can deduct all your stock losses instead of being limited to $3,000/year. So if your wife made $100K working and you lost $100K daytrading, you'd have no income and get a big refund for all the taxes your wife paid in. Also, the wash sale rules no longer apply if you make the election.
3. You also need to file an IRS form 3115 (change in accounting method). Why? Because you used to report your stock trades on Sch D as capital gain/loss and were limited to a $3K loss but once you elect mark to market you report your stock trades on Form 4797 as ordinary income/loss and you can deduct all your losses. That's a significant change in accounting method.
4. You elect mark to market by drafting a letter in MS Word and attaching it to your tax return or extension. In this letter called a 475(f) election you attach a statement that includes the following:
1. That you are making an election under section 475(f) of the Internal Revenue Code.
2. The first tax year for which the election is effective.
3. The trade or business for which you are making the election.
4. That the election only applies to securities and not to futures.
The statement should have been filed by April 15, 2009 for your 2009 tax return.
5. If you're going to form a business entity for your trading I recommend a S-corp. A C-corp has double taxation and an LLC pays high gross receipts fees. The benefit of forming a business entity is that it solidifies your business activities and expenses with the IRS. Running a $100 million in stock trades through your personal tax return is a big red flag to the IRS. Your new business entity will also need to make a mark to market election but does not need to file a form 3115 (b/c there was no accounting method you changed from).
6. Whether or not you made a mark to market election, you can write-off some expenses on your Schedule C of your tax return such as internet, WSJ subscription, stock seminars, margin interest, etc.
7. More info on daytrader vs. investor can be found here:
http://www.irs.gov/taxtopics/tc429.html
I don't want to answer everyone's PMs so don't send me any PMs.