Bitcoin is a great choice for long term investment. So you can try a method which is known as "Systematic Investment Plan" or SIP in real world and used in stock market trading a lot. It uses dollar cost averaging method to lower your portfolio risk. Decide an amount to invest in bitcoin at a certain interval. Like invest $100 bi-weekly in bitcoin. So it will help you to buy bitcoins at different price points which lowers your buying cost by averaging the value of your holding. You will sometime buy at high price and sometime at low price. This method is actually very helpful for a fluctuating market like bitcoin.