As time goes on, unused, it just gets tougher and tougher to mix.
When new,the tried and true wood stirrer stick usually works.
After a "few" years: stirrer stick to break it up enough so you can use an electric drill mixer... tedious, but $$$$ saved.

The big keys to multi year use are:
Don't let it freeze ( already mentioned)
Clean the sealing groove as clean as you are able and make sure you tap the lid back on square and all the way down.