Alright, I am assuming you are playing slots for the Lucky Coin fragment and not because you like it (because I personally found it to be a bore and a chore). This is what I did.
I always started with at least 5000 coins. I walked up to the lady that tells you the mood and saved my game. If she doesn't say something like, "It's like a warm summers day" or the machine is "On Fire" I would just quit to the main menu and load up my save again. Eventually she said the above and I started my grind, thus begins the boring part.
I used a small piece of tape to keep the button down to keep the machine chuggin' along (watching a movie while it was running :P). Hopefully you will hit the Jackpot, which will prompt the machine to enter Overbreak mode (I think this is what it was called...). In Overbreak mode you have something like ten times the chance of hitting the jackpot and bring in 5,000 coins per win.
If you can make it to Overbreak mode you should get the achievement for 10,000 coins, if you don't already have it, and get your Lucky Coin fragment.
If you don't get it and run out of coins, don't panic... just reload your game and try again!