There are different approaches for that based on different assumptions and based on which risk level you prefer. My favorite strategy is the one outlined here: https://bitcointalk.org/index.php?topic=345065.0 (here's a calculator for it: https://jlopp.github.io/bitcoin-savings-plan/).

Basically every time the prices increases by X% sell Y% of your investment and don't touch the remaining part unless it is X% higher than the previous peak at which you sold. This ensures that you're realizing gains on the way up, but still benefit if the price continues to climb. In addition what I like about that strategy is that it takes emotions out of investing: Decide once what your parameters for the strategy are and then just strictly follow it, whatever happens. It's a lot easier to stomach a 90% price decrease if you realized a sufficient amount of gains on the way up.