@Jack B. I'm a firm believer that equity (in a property) should be put to work and make itself profitable. This is not the case if it "sits" in the property.
While you obviously want to have a decent equity-cushion (@David Faulkner's point #1) the only way that I can think of to protect any equity that exceeds this "cushion" (in my opinion the percentage is to be defined by each investor individually based on their situation) is to pull it out and invest in something that creates value.
This can be another (cash flowing) property. Or maybe the stock market with a decent yield on investment (diversification!). Then again, both of these are currently somewhat toppish. While one will find real estate markets in the US that are less toppish than others, one might not be able to or not want to invest there for various reasons. I'm in that boat. And in "my" markets I'm having trouble finding good deals when trying to follow the usual "number crunching".
So now I have a bunch of cash sitting in my account that costs me serious money in interest payments. Yes, I can deduct that from my tax bill (important thing for my situation) but ultimately it currently costs me money. Period. Then again, I did this to be ready to pull the trigger if a good deal shows up. And there will always be good deals at some point. Just got to be patient.
So now I'm looking into possibly using these funds for hard money loans to others, playing the interest rate spread game. This is usually short term and therefore, say over a period of 6 months, I should have covered my (annual) cost and possibly even made some money in the end while also having a half year gone by to see what the market has done, with new developments (liitterally) having surfaced.
One other thing that just now occured to me: what about investing that money into an empty lot. Lot's seem to increase more quickly in my area than some properties with structures on them. Go figure. But to a certain degree it makes sense: a building requires upkeep (= costs money), a lot does not (leaving aside some minor costs for e.g. infrequently checking on it). These lots are sought after (obviously depending on the location - as always), so selling it fairly quickly should not be an issue if the money is needed for a good deal. Maybe something to look into in more detail.
Otherwise I fully agree with what David Faulkner wrote.