You only owe taxes on your net winnings. If you can prove that you lost more than you won, than it will turn out to a "0" and you will not be paying taxes.
Example.........Say you go to Vegas and play Keno at $20.00 a ticket and you lose the first 100 times you play (you'd better keep the losing tickets)......You'd have lost $2000. On that 21st ticket you win.........$1500.................that is still a net loss of $500 and you won't be paying taxes. Its all about keeping losing stubs and claiming your winning tickets. If the winning ticket is large enough, many times the taxes will be taken out and of course the IRS will know about it! Again, the most important thing is to keep thiose losing tickets.
I knew a guy who won $2000 at the horse track in a trifecta and taxes were taken out (so we know the IRS knew he had won some money) so he spent the next two weeks going there and picking up losing tickets off the floor so he could offset that $2000.
If you do it for a living, you should keep good records.
As for offshore wagering......I'm not sure how or if you would want to show your winnings or loses because of that gray area concerning gambling in most U.S States.
I could talk all day on this subject from exeperience, but I'm tired and its getting late.
Wager2Win