Probably the best way to grow your Bitcoins is to save them and wait for about a year. By then the price will probably have gone up by then and you will have a substantial amount of money.
But what if the bitcoin prices are fallen after a year, this is little risky idea. I would suggest to go for other ways to earn more bitcoins from mining, trading, online work wherein payment received in bitcoins - visiting websites, viewing advertisements, signature campaign and you can buy bitcoins also. So these are some safest way to earn and grow your bitcoins. Don't think about gambling to earn the bitcoins, you may get some coins in the beginning but at the end you will loose your money.