[partsnut]

As far as I know you have to file as a "Professional Gambler" as your occupation and primary source of income.
Otherwise your losing tickets will not be acknowledged.
As Gary stated and which is well advised, you must keep very detailed records of your expenses as well as your win and loss records which is not difficult to do when us use a service such as Twin Spires. If you play at the track or at an OTB office. Photocopy your win tickets and keep all your losing tickets.