A very good way to earn some btc is to join signature campaign. Also you can start trading but you must be very careful if you do not have any experience.Whatever you do, avoid these bitcoin doubling sites you will lose your money in these scams.
Signature campaigns are really best way to earn some money, but in there you do not double your bitcoin, but in theer you get some amount of money for your work(marketing) for them. while in trade he can double his coins, just like I doubled my money when I invested them in LIR.