Perhaps the most tested way to double your money over a reasonable amount of time is too invest in a solid, non-speculative portfolio that's diversified between blue-chip stocks and investment grade bonds. While that portfolio won't double in a year, it almost surely will eventually, thanks to the old rule of 72.
The rule of 72 is a famous shortcut for calculating how long it will take for an investment to double if its growth compounds on itself. According to the rule of 72, you divide your expected annual rate of return into 72, and that tells you how many years it takes you to double your money.
Considering that large, blue-chip stocks have returned roughly 10% over the last 100 years and investment grade bonds have returned roughly 6%, a portfolio that is divided evenly between the two should return about 8%. Dividing that expected return (8%) into 72 gives a portfolio that should double every nine years. That's not too shabby when you consider that it will quadruple after 18 years.
