I and many of my friends have our own mantra before and after we gamble, I just discovered this when we have a drinking session we find out that many of us have our own mantra before, while playing and after or even in the middle of playing.
Mine was "This is my lucky day and I'm claiming it" and when in the middle of the game "I need to play money that I can afford to lose".
How about you do you have one and when do you recite it before, while playing or after you gamble.
Afaik mantra is a kind of ritual or a superstitious beliefs. Playing or spending what you can afford to lose is just a golden rule but I don't think it is qualified as a mantra tho the first one you said, the lucky day thing can be considered as one. There are many more like wearing a certain color of t-shirt, praying before or in the middle of betting, and so on.
Lately I've been gambling carelessly. No not the one you are thinking of spending recklessly but I mean is I don't mind losing or even winning, this allows me to gamble calmly and with less to no regrets at all. With that said, there is no need for me to do any mantra's but before, I always pray that I will win. It sometimes work but sometimes don't.