The secret is to have a lot of money already. Bitcoin has a lot of profitable investments in it, and it also is a profitable investment. If you have a lot of money, you can easily get 20% a year or so through popular investments like casino bankrolls. That'll get you 0.1 BTC per week if you're prepared to basically go all in.
That is a risky move to make to invest huge sum of money only in bitcoins and investing in casino bankrolls is also an risk as you never know how long they will share profits with you and they may shut down their sites in long run so its better to earn them instead of taking risk of investing.