According to me The best ways to grow your BTC is by trading. You can multiply your principal amount in few days of trading and your profit margins can be terrific. But remember that trading is a risky and unpredictable job.
You can also grow your Bitcoin by investing it in various new projects.
It is true thats the best option people had and the safest one but when you are in hurry to double your bitcoin you need risk it from pmaying gambling this is also the fastest one when it comes of doubleing your bankroll. All money involved in money marketing are highly risk coz if you don't risk money theres no profit waiting for you.