Store passphrases in your keychain
On MacOS (and OS X) there is a handy option for
ssh-add to store your key's passphrase in your keychain.
ssh-add -K ~/.ssh/name-of-your-key-here
You will be prompted to enter your passphrase after which it will be stored in keychain.
Load passphrase from keychain on MacOS by default
In MacOS (Sierra) your ssh key's passphrase is no longer automatically retrieved from keychain across sessions.
This means that you will be reprompted for the passphrase if you restart. Being required to re-enter your passphrase each time could be the behaviour you want for added safety, but if not we have a solution.
Add the following to your
Host * UseKeychain yes
This was originally answered in a StackExchange post but I've replicated it here for my own easy access.