Use a key derivation function with a variable number of rounds, such as bcrypt.
The passwords you encrypt today, with a hashing difficulty that your own system can handle without slowing down, will always be vulnerable to the faster systems of 20 years in the future. But by increasing the number of rounds gradually over time you can increase the amount of work it takes to check a password in proportion with the increasing power of supercomputers. And you can apply more rounds to existing stored passwords without having to go back to the original password.
Will it hold up for another 20 years? Difficult to say: who knows what crazy quantum crypto and password-replacement schemes we might have by then? But it certainly worked for the last 10.
Note also that entities owning supercomputers and targeting particular accounts are easily going to have enough power to throw at it that you can never protect all of your passwords. The aim of password hashing is to mitigate the damage from a database leak, by limiting the speed at which normal attackers can recover passwords, so that as few accounts as possible have already been compromised by the time you've spotted the leak and issued a notice telling everyone to change their passwords. But there is no 100% solution.