Sad but true: None of the above methods have a chance of working. Strong encryption, separate storage of data media, total destruction of the computer -- zero, zilch, nada, nothing, zip, gornicht. These are all excellent hi tech countermeasures -- but the problem is lo tech, not hi tech!
ThinkGeek.com will sell you a keystroke logger (the "Key Katcher", currently only $79.99 + shipping!) in a small plug. If I used one on your computer, you (being paranoid) woud notice it because it's too big -- it contains two plugs. How big do you think these have to be? How small could TheBooBooKitty make one?
It's really that easy. In the ongoing case of Nicodemo S. Scarfo, the FBI did even less: it appears they installed key logging software on his PC, to capture his PGP passphrase. Hardware would doubtless also be doable (perhaps requiring some additional effort, i.e. at greater expense). Nothing in PGP or in security precautions helps against this type of attack -- computers not doing "what they are supposed to be doing".
You can achieve some measure of security, e.g. if you post a guard around your computer at all times. Assuming no wired (or etherial) access to your computer, the only problem is that of physical access to the system;
a highly-motivated guard can significantly reduce the possibility of unmonitored physical access. (Hopefully not too) Late addition by APE'EM (the Association of Paranoids for Ensuring Effective Means): It doesn't help to post highly-motivated guards. You'll have to ensure they are extremely highly paid and have no blackmailable spots in their past -- or the FBI will just pay them to look the other way. We're talking lo tech, after all!
Or, of course, you can do what I do: I don't live in the U. S. of A.. The FBI doesn't try to read my data. The CIA (among others) do that.
Also, don't forget to wrap tin foil around your head at all times! They can SUCK the passphrases out of your brane with an electro-magnetic vacuum cleaner!