Doing your best probably isn't enough. To have any hope you'll have to hire expensive security people and buy lots of insurance.
All you need in order to be exploited is to be using software with 0day exploits. Many known exploits are not public. In a very real sense, you are only protected to the extent that you are a small target.
As the potential payoff of a hacker approaches $1 million, the likelihood of being hacked approaches 90%. Software really is THAT insecure and bitcoin thefts are not prosecuted making it basically risk-free to steal bitcoins.