TrueCrypt is an excellent program, +1 on the advice above.

That being said, I wouldn't trust a single USB key, as most of them use really low-end NAND flash memory. Stick to good brand names (SanDisk, Crucial, etc.) and if it's going into a deposit box, use several USB keys for redundancy.

If not written much and if kept at reasonable temperatures, flash will hold data for a very long time (10-50+ years) but if stored in a hot attic in Phoenix, it may not last even a year.