The best way (only-way perhaps) is to encrypt your sensitive data. A loopback device or a physical partition with LUKS should work well, as long all sensitive data are stored there. Encrypting the whole /home is not a simple thing for N900, so it could better work with the cooperation of programs that store sensitive data.

In any case, as a starting point, infrastructure for a password manager seems like a must these days. Encrypting the stored passwords is a first (and needed) step. I'm describing something like kwallet.

However, if you're considering the authorities, I suggest that you don't underestimate the point of the following sketch: