I think if one is going to sell it's prudent to do so when the price is at a peak, not like it is now - at a bottom.
This is the general rule that it is easier said than done.
Like in stocks, there are certain indicators that will help you arrive at a decision whether to sell/purchase more BTCs in the process. But before you arrive at a decision, always be definite. Only purchase BTCs that you are willing to lose in the process. You can also sell 60% of your BTCs the moment you arrive at profit and keep the remaining 40% for long-term investment.