Bitcoin's price depends on the supply and demand in the market. The demand does not go down easily so we cannot deduct demand from it so instead, we increase the supply. We've known the law of supply and demand so we should use this concept to manipulate the price. Increasing the supply would definitely make the price to decrease. And the best way to do this is to sell your coins. Not to anyone but to the exchanges. Minimal amount may not affect the price that much so a big amount will do the trick. Goodluck encouraging others to dump their coins, you need luck after all.