If you are really serious about this, then you should consider building a special computer just for the purpose of viewing and manipulating your sensitive data. A computer without a hard drive. Actually for best results, you will have a hard drive, but it will only be filled with a decoy operating system (Microsoft Windows would be best).
Here is what you will need to get started.
First you will need a computer that can boot from CD-ROM, a TrueX or other fast drive would be a plus, since this drive will be used extensively.
Next you will need to buy one of those removable hard drive caddies that install in a 5.25 inch drive bay. This is where your decoy hard drive is going to be installed.
Next you will need to buy some RAM, a lot of RAM, 1 GB or more is preferable, but 768 MB will also work nicely.
Next get yourself a Superdisk drive, or a Zip drive, this is going to be your only permanent storage, so choose wisely.
Finally you need to make yourself a bootable CD-ROM of your favorite operating system. The easiest way to do that is to pick up a Linux distribution that comes with a "Distro on CD", that includes all the basic tools, and even X-Windows on a bootable CD. Bootable Windows CDs are also quite possible (do a web search, you will find the information).
Now you need to put all of this together. Install Microsoft Windows on your dummy hard drive. Put some games on there, add a few innocent websites to your favorites, and generally make it look like it gets used. After you have this looking good you should pull that hard drive caddie out just a little bit, so it is no longer connected. This is to ensure that you cannot possibly access this drive from your real system.
Your real system is going to boot from the CD-ROM drive, and use a ramdrive for storage. The Zipdrive (or Superdrive), is for making a daily backup of your ramdrive (or whenever you turn off the computer). Your daily backup should be heavily encrypted, and it is very important that you only keep a single disk, never use another disk. You are going to be keeping this disk on your person at all times, you do not want to leave it lying around where any stray FBI agents can discover it.
You should next rig a hidden security switch for your computer. This can be as simple as a hidden switch that shuts down the power if the door is opened. But I have a more elegant solution. My solution requires five switches. The first is a pressure switch underneath the case of your computer, wired so that moving the computer (even slightly), will turn it off. That, however, is not your main protection. Your main protection is going to be inside your monitor. You are going to crack open your monitor case and wire up a set of switches to the monitor power button, and one of the other buttons on the monitor. You are going to wire them in such a manner that if you turn the monitor on, it will turn the computer off, unless you also press the second button as well. You can flip your switches everytime you walk away from the computer, if someone walks up and turns on the monitor, the system will shut down. Finally you are going to put another pressure switch under the monitor, and then another in the back where the monitor plugs into the video card. If anyone messes with this computer in any way it will shut down, erasing everything in its ramdrive, leaving no evidence at all.
Now you have a working system, that does not hold any data after it is powered off. You should practice your emergency measures (or what to do if the FBI really does show up). Basically all you have to do is eject your system disk (and put it with the rest of the system disks that it came with), push that hard drive caddy back in place, and make your backup disk vanish.
The only problem is making your backup disk vanish. There are several solutions to that problem. The first is to have a whole box of disks all written with the same file name as your real backup disk, and encrypted data. Simply toss your disk in the box, and hope they waste years trying to decode the information on the wrong disks. You are going to want to handle all of these disks frequently, so your fingerprints won't give away the identity of the true disk.
The next solution is to eat the disk. You don't have to eat the entire disk, just crack it open and pull out the little black disk inside that holds the actual data, and eat that. The final solution is to burn the disk. Keep a little torch on your desk, and melt the disk at the first sign of trouble. Burning the disk is safest (as far as data recovery goes), But eating it, or tossing it into a box of decoys can be accomplished much faster (you know, while they are breaking down the door).
Or, if you are less serious about hiding your data (or more serious, depends on how you look at it), you may want to consider the following tips.
First of all, never use text files, they are the obvious first place to look for your information. If you do use text files, then encrypt them, zip them with a password, then rename the zip file to a .sys file (or other system file), and hide it somewhere in your system.
Do you remember how the evil Professor in Maniac Mansion hid the lock combination in the high score table of the video game? Well you can do this as well. Install MAME on your computer, and play all the games at least once (you may want to have a batch file do this, as there are nearly 4000 games). Then pick a game with a nice juicy high score table, and play it over and over again until you have entered all the needed data into the high score table. Then copy your NVRAM folder, uninstall MAME, and then reinstall it. Your data is safely stored in the high score table of one of 3800 games, with little chance of anyone finding it.
Hide text data inside of image files. There are several ways of doing this. The first is simply to open up an image file with a text editor, and type your text right in the center of it. This usually won't kill the image file, so it will still display. But this method is very easy to spot if the file is examined. The next way is to hide the text inside the actual picture. Pick a nice juicy picture, and zoom in to it at the pixel level and draw your message right into the picture itself. You would be wise to do it in a color that is visually identical to the background. The text will be all but invisible, but you can pull it up using an image editing program, but only because you know exactly where it is. The final way to to this is to make an animated gif that hides the data in an undisplayed frame, you can then get your data back by opening up the picture in an image editing program (this technique does not hold up well to scrutiny). When using any photo technique it is important to hide your real pictures in with thousands of decoys, to further slow down any examination of them (you might as well encrypt them as well). You may also want to consider running filters over all your images, filters that you can reverse later to get your original picture back (this is kind of like encrypting a picture).
You can also effectively hide data inside of sound files, mpeg files, and all sorts of other multimedia files. It can be as simple as hiding your 15 second audio message inside of one of 8,000 MP3 files, and hoping they don't bother to listen to all of them (a good way to do this is to record your message at a miniscule volume level at the end of a track, it would just sound like dead air unless you turned the volume all the way up.
Be creative. You can hide your data inside of source code, inside config files, you name it, and you can hide it there. You can even hide your data inside of E2 nodes (comment it out, and it doesn't appear to normal readers, yet it is still there when you go to edit your node).
If your data is truly of earth shattering importance, you may want to consider combining every method I have mentioned (and the ones in SharQ's writeup as well). How far you go, is up to you.