Write it down in a paper then fold it to the size of your wallet. Just make sure to not lost your wallet.
Simple yet very effective. I'll add to this one, create many copies as back up maybe 2-3 of it. Take one copy always with you on your wallet/pocket while the remaining copies keep it somewhere that no one knows. It's fine to keep your data's on your PC but make sure that you don't click malicious websites or download software that you are not familiar with.