When the system is being openly discussed or sold on the Internet. I can think of no better way to negate the utility of any of my systems than by discussing them with others.
And if I had a system that didn't make any money then selling it would be the only way of increasing its utility, which is the only reason I can think of for selling a system.
By all means discuss the mathematics of your system, if you don't understand something yourself. But make sure you don't give away the workings of your system.
Visit any of the many gambling forums and you will see beginners announcing a wonderfully simple system that is going to make them rich. Only to have it shot down in flames by someone who understands probability theory.
Online poker brought thousands to the Internet, believing they were going to get rich. That spawned websites for people to discuss strategy (thus negating the strategy's utility) and then came the software (The Head Up Displays) to give you an edge over everyone else.
Likewise the betting exchanges have brought thousands to the Internet who think they are going to become expert traders. They get the software, put money into their account and then lose it.
You need a solid base of probability theory to understand why that wonder system doesn't work. Learn how betting markets work and how they are bounded by the overround.
Arbitrage tells you why prices don't behave the way you want them to, otherwise we would all be rich.
Get spreadsheet software to test your theories. Make models of when it all goes wrong! just as much as when it all goes right so that you understand the downsides.
Above all, keep your systems to yourself otherwise anyone you share your system with will share your potential profit with themselves.
