Gambling losses are only deductible up to the amount of the wins and if you are itemizing your discounts on Schedule A. Also, you need to be able to substantiate your wins and losses by keeping wagers, tickets, or any records just in case you get audited. You can use the following items for record keeping purposes in case you get audited by the IRS:
Lotteries : (scratchers, super lotto, mega millions, keno) winning statements, unredeemed ticket purchases, payment slips, W2 G form, losing tickets, etc.
Table Games : (such as poker, black jack, roulette, craps, etc.) casino credit card data, the number of the tables played. Most people have a club card that can accumulate credits for free meals, complimentary rooms, etc when you've been playing for a while. This clubcard can provide you the times, dates, amount of wins or losses, and the number of the table that you are playing when you request for a print-out from the table manager.
Racing : amounts collected on winning tickets, unredeemed losing tickets, payment records from the racetrack.
Slot Machines : (wheel of fortune, poker, etc) Record of the machine number and the dates and times of all wins, receptions of wins from the machine or from the cashier.
Bingo : cost of tickets purchased, amounts collected on winning tickets, any receipt from the casino or place bingo sweepstakes are held
Sports Bar : (parlays, straight picks, quickpicks) ticket wager showing the games played and the cost, amount collected on winning tickets, other supplementary records such as winning statements.
Additional information about this tax topic can be found on the article "Can I Deduct My Gambling Losses?" on the Filam Personal Finance website.