If you're looking at a horizon of 3-6 years, Gold or other precious metals is where you want to put your money. This is the best way to protect yourself against inflation in my honest opinion. Gold historically has been a pretty conservative bet.
There is no sure fire, legal way to double your money in any get rich quick scheme. Hard work, though, generally prevails, so the best advice is to look for a way to improve efficiency in what you do and then capitalize on that learning. If there is something which can be done better, do it.