Say for example you have £50 in your pocket and also access to other funds, but you really don't want to touch them. Before you approach any machine, say to yourself that you want to walk away when either of the following two situations occur:
1. You are a tenner up.
2. You are a tenner down.
Of course, the top one is by far the better situation, but the bottom one isn't as bad as you may think.
Let me elaborate.
We're all pretty good judges of machines, so within the play that you get within your first tenner, most of the time you should be able to judge how things are looking. If you're £10 in and don't even get a board, then get the hell out of dodge! That £10 could soon become £20 without a comeback of any sort! Of course, the nature of gambling means that the following thoughts start to form:
"Crikey, £10 without a board, it must give me one in the next few quid?!"
"Ah, it's saving up for a big win on the next board when it finally does give it to me!"
"This is just the calm before the storm!"
How often are those three thoughts not the case? I'd say 75%+ of the time.
If you walk and you're a tenner down (from £50), you've still got another 4 machines that you can perform the same routine on. Now unless you play in Burntland Road, Old No Luck Town, Already Droppedshire, at least 1 in 5 of those machines is going to return some sort of profit?
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Having said all that, I'm speaking from a point of view that playing is about <15% of what I spend my time doing (I work full time, have a life outside of the pub, etc.) This wouldn't really work for those who 'do it for a living'.
Keeps me happy and ticking over nicely on a long Friday pub crawl!