make an encrypted disk-image with diskcopy, and store all the files you want to protect there...
make the image so it's big enough, and read-write so you can later add what you wish, do not add the password to your keychain.
You can just change the files / folders permissions. example I have a couple ummm movies I rather not let anyone else see. What i did was change the access right of the folder they are in to "no access". When I want to go look at the movies I just change the access of the folder back to "Read".
I guess there are several ways to do this depending on how you and other users log on.
It is also easier to do using 10.2 since they improved the get info window.