Once you start they'll audit you for the last X number of years . . .
http://www.irs.gov/publications/p529/ar02.html#en_US_2012_publink100026911
Gambling Losses Up to the Amount of Gambling Winnings
You must report the full amount of your gambling winnings for the year on Form 1040, line 21. You deduct your gambling losses for the year on Schedule A (Form 1040), line 28. You cannot deduct gambling losses that are more than your winnings. Generally, nonresident aliens cannot deduct gambling losses on Schedule A (Form 1040NR).
You cannot reduce your gambling winnings by your gambling losses and report the difference. You must report the full amount of your winnings as income and claim your losses (up to the amount of winnings) as an itemized deduction. Therefore, your records should show your winnings separately from your losses.
Diary of winnings and losses. You must keep an accurate diary or similar record of your losses and winnings.
Your diary should contain at least the following information.
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The date and type of your specific wager or wagering activity.
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The name and address or location of the gambling establishment.
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The names of other persons present with you at the gambling establishment.
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The amount(s) you won or lost.
Proof of winnings and losses. In addition to your diary, you should also have other documentation. You can generally prove your winnings and losses through Form W-2G, Certain Gambling Winnings, Form 5754, Statement by Person(s) Receiving Gambling Winnings, wagering tickets, canceled checks, substitute checks, credit records, bank withdrawals, and statements of actual winnings or payment slips provided to you by the gambling establishment. For specific wagering transactions, you can use the following items to support your winnings and losses.