To change the machine, just save at the woman you talk to, if she doesn't say "it's like a warm summers day or that machine is on fire" just quit to main menu, reload and talk again. Once you get the desired mood it's all pure luck.... As said above manual seems to be better than auto play. Other than that it's just pure chance >.>
The only reason to play slots is if you're going after all the fragments in the game, which is one of the most tedious tasks in any game ever. You get 1 fragment for winning more than 7777 coins in slots. Otherwise, I would recommend just earning Gil later in the game and buying whatever you want from the casino with Gil.