To lock a folder so it needs an admin password for access:
1) get info on the folder and turn down the "Details" triangle at the bottom
2) set group to "nobody" and no access
3) set others to no access
4) change the owner to "nobody"
5) close the lock
Note 1: There is a window of time of about a minute when you can still change these privileges without entering your admin password. To gain access back to the folder click the lock and select your normal user. You will then be prompted for an admin password, type it in and you now have access again.
Note 2: I tested this using my normal everyday non-admin account. It should work just the same if you use an admin account as your regular everyday user account, but you should just double check the procedure if you do. In fact, if you use an admin account as your everyday account then you probably want to change that anyway.
Note 3: This is not something I've rigorously tested, so (unless someone wants to do some extensive testing) it is not something you should rely on for maximum security. This is probably best for keeping the casual snooper from easily getting into your private stuff. For instance, the files names will still probably show up if a person does a find file command.