What I did when I stored bitcoins in my head was create a passphrase and a pin number.  The pin number represents the number of times to run SHA256.  I now only remember the "first bits" to the public address, the passphrase, and a pin.  I have my savings account now that I've confirmed it several times with different pass phrases and smaller amounts.  To avoid the change problem I always send the entire balance out, and send change back in manually.

I think it's pretty secure.  Nevertheless it is very easy to hack with a rubber hose... just saying.

Anyone know of a calculator for passphrase entropy?

j