I have used a similar method in the past: I had a plain-text file containing my credentials and encrypted this with the Blowfish cipher.
Now I use KeePass, an offline password manager. I strongly recommend it, as it is
- Much more user friendly than an encrypted text file, and
- Much more secure than a text file encrypted just once (N = 1, see below). KeePass supports multifactor authentication and protection against keyloggers, just to name a few advantages.
The encrypted database containing my credentials are stored in a directory that's automatically backed up to Wuala, a secure file storage service.
I also recommend to use a key file, which is like a second, very strong password (at least 256 bits of entropy), and store this on a USB mass storage device (something you have). Combined with a strong master password (something you know), you've essentially set up a 2-factor authentication mechanism that's very difficult to break.
Be sure not to save the database and the key file in the same location. It is essential that you keep the key file secret, so on a medium that's on your person, and a backup (e.g. on paper) in a vault or similar secure location. If you lose the medium that the key file is stored on, you can simply use KeePass to generate a new key file, rendering the lost key useless. And even if an attacker could obtain the key file AND the database, the database is still protected with your strong master password.
KeePass makes it actually much more hard to break into the database by brute-force or dictionary attacks. It does this by encrypting the database not once, but many times. The recommendation is to set the number of rounds N so high, that decrypting the database takes 1 second on your system (on modern hardware N easily exceeds 10 million). This might not look much, and indeed it's not for day-to-day use, but for an attacker this will make the brute-force or dictionary attack take N times longer.
See KeePass Security for an explanation of all security features.
You can also use a YubiKey instead of the key file. KeePass supports the YubiKey through the OtpKeyProv plugin. In fact:
All generator tokens that follow the OATH HOTP standard (RFC 4226) are supported.
So you're not limited to a YubiKey.