Bitcoin has a long history of steep dips, yet it has always gone higher. I think it is prudent to never sell below ATH.
Same applies to Monero of course.
Don't invest in BTC/XMR any money that you cannot lose. And sell so much in the tops that you can take the dips to zero if need be, without getting tempted to sell at the bottom.
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In the normal course of speculative play, you must start out with a willingness to be
hurt, if only slightly. Bet amounts that worry you, if only a little.
Another truth which needs highlighting. I'd add a few items though:
a) specialize on a particular game/industry. Learn it inside out - increase or odds by making educated predictions.
b) hope for the best, plan for the worst. That is assume you will be hurt in ways you don't know yet.
c) know your league. You cannot play the billion dollar game with a 10k portfolio.
Bitcoin is an interesting phenomenon. It seems to negate point c), since the access to bitcoins is leveled. Unfortunately it also enhances point b), b/c the irreversibility of the payment makes it hard to contain fraud.