Gambling is probably the safest and most effective method. Double or nothing you'll have a 50/50 chance of at least doubling your money!
Firstly your are saying that it is the safest and most effective method and then you are saying you have 50/50 chances of winning and losing, then how come it is the safest? Though gambling can make you rich in one bet if you win it but the question is are you really going to win that one?