I currently store my seed in a plain text file, backed up on a USB drive, which is a security risk. What is the safest way to store my seed since there is no IOTA hardware wallet available yet?
For digital storage, I would recommend using an offline password manager such as Keepass which can encrypt your key behind a password.
For paper storage, it may be good to print out a copy of your seed in a QR code or otherwise stored and printed out and kept in a safe or safety deposit box.
For hardware storage, hopefully options arrive soon such as Ledger or Trezor.
What works best for me is storing the seed encrypted in a KeePass database file (or the offline password manager of your choice) on my computer and on an USB-Stick in another place (backup in case of fire, theft, ...).
Having IOTAs on a Ledger Nano or Trezor would be great and they (Ledger or Trezor or both) will add IOTA eventually.