Agree about paper wallets or some digital version of it, i.e. some encrypted file containing all your private keys. In general - for long term, store rather keys than wallet files. This way you won't be vulnerable to future versions of your wallet software being backwards incompatible or totally nonexistent in a few years.

Splitting funds into several wallets is a good idea, but remember to keep these wallets in separate places as well. There's not much advantage of having bitcoins splited over five wallets if you store private keys for all of them in one place, so if it becomes compromised, all your wallets will be at risk.

Also, always store multiple copies of your wallets in different geographical locations. There are fires, earthquakes and all sort of stuff than could destroy your data.

There's one more thing to consider when storing wallets/keys for very long term. You probably want them encrypted using some strong passphrase and if they are proper cold wallets, that means you won't access these keys for years. You'd better make sure you'll still remember your 50-chars long encryption passphrase five years from now.