ive lost many due to my own stupidity over the many years.. testing new wallets without understanding them enough, not having copies of seeds, having a couple paper wallets deteriorate, not maintaining proper backups.
best masterpasw application is old paper notebook and
ink pencil .
the typical paper notebook is pretty low quality paper. ink blurs and washes out. use high quality archive paper and pencil (doesnt wash out typically) or permanent marker (test it 1st companies seem to differ on whats "permanent").
better: stamped high melting point metal of some sort.