The truth is that there is no trick to slot machines. They provide random results based on set mechanics and it all comes down to luck. With that being said, not all games are the same, so picking the right options is key, and you can still change the size of the bet throughout the session for better results.
In short, you want to have a larger bankroll than the figure that you ultimately come up with. To be on the safe side, you should have $350 or more instead of justR$250.