Preparing for the worst
Bitcoin is increasing in price so fast that it's pointless given the current price - it will be completely wrong by the time you are reading this. Let's just say it is currently at an ATH (which will probably still be true when you read this).
The market has gone past exuberance and entered hysteria. We have been on a year long bull run that seems to have no end in sight.
However, there are reasons to be fearful. I have previously written reasons as to why we are not in a bubble. I now believe Bitcoin is in the midst of a bubble, and is taking the rest of the market with it. Large cap projects such as IOTA, Dash, Monero and others have seen increases of 1-10x their market cap in week long periods. This is fairly unprecedented. Bitcoin has essentially doubled its market cap this week, adding c. $140bn. That is staggering. The mania is everywhere. I have been inundated with calls and messages about it. I have heard multiple sets of people discussing it at work, on the train, pretty much everywhere.
I do not know how much longer this will last for. It could conceivably last for months or years. However, I believe that we will see a big crash (c. 70-90%) in the coming weeks and so think it is prudent to take steps for protection.
Zero cost basis
If you have been in the market for longer than a couple weeks you should really have withdrawn enough to be operating on pure profits at this stage. Not having any skin in the game makes it easier to make objective decisions and protects you against losing it all.
Withdrawing to fiat
I am not recommending withdrawing everything to fiat - this bull run could just be getting started - but there is no shame in withdrawing 25-33% of your stack back into fiat. The reasons for this are twofold. Firstly, you protect yourself against losing it all (because when the crash comes it is going to come quickly). Secondly, you provide yourself with the resources to buy back into any big dips. There is nothing more frustrating than not having money spare to buy market overreactions as a result of being fully invested.
Assess your own circumstances
Are you close to retirement? Do you need this money for something specific? Have you already mentally cashed in your gains? Obviously these initial three points will vary depending on the amount invested/your current portfolio, but think carefully and clearly about how you would react if your portfolio dropped 90% tomorrow. There is no shame in taking some profits. Profits are good. Don't let greed get in the way.
Move to safety
When Bitcoin sneezes the entire market catches a cold. The difference this time is that Bitcoin might not just sneeze, it might catch pneumonia. That said, if you are preparing for a crash then now is the time to be in the strongest projects. Bitcoin, Ethereum and Monero are all solid projects. Ethereum has also not seen the huge upsurge of others in recent months and so could be better protected against a large crash compared to others. Be very careful which altcoins you are in. These typically bear the brunt of market losses. I would also be wary of large cap coins which have risen quickly in recent weeks and likely haven't established support.
Have contingency plans
The exchanges are not able to cope with current demand - they will fall over the second a true flight from the space begins. Make sure you are registered on multiple exchanges - do not rely on just one. Learn how to use LocalBitcoins - it can be a life saver for those times where the exchanges aren't operating.