Don't listen to anyone who tells you it is difficult to learn to sharpen an x-acto blade or not worth the effort. It is very easy to learn how to put a wicked edge on an x-acto blade, and once you learn how to do it you will no longer think than a brand-new x-acto blade is sharp.
Sharpening tip number 1:
Use wet-dry sandpaper and small piece of plate glass as your sharpening tools. You will want a range of grits between 200 and 2000. I use 320, 600, and 1500 because that is what I have. Always keep the paper wet and it will stick to the glass.
Sharpening tip number 2:
Make yourself a strop. Get a piece of raw leather and rub some chrome green polishing compound into it. Raw leather is getting more and more difficult to find, and in a pinch you can take an old belt and rough it up with some 100 grit sandpaper. Chrome green is best, but just about any polishing compound or metal polishing paste will work in a pinch.
Sharpening tip number 3:
Sharpen the blade as little as possible. Only the edge is hardened, and you can sharpen away the hard steel. Keep the strop on your bench at all times and use it frequently. Just a few light passes will do wonders. Remember: when using the strop you lead with the heel and drag the edge.
Sharpening tip number 4:
The most common sharpening technique is not actually sharpening, but rather re-pointing. Drag the heel of the blade across 200 - 320 grit sandpaper to form a "drop point tip." A lot of people have difficulty picturing this. Think about how break-away blades work to expose a new tip. You are doing the same basic thing here, only removing far less blade material. After re-pointing, touch up the tip with 1500-2000 grit sandpaper. Just lay the blade face flat on the paper and draw it towards the tip, lifting the blade slightly towards the end of the stroke. What you are doing here is removing the rough edges created by the 200-320 grit sandpaper. Then hit the strop a few times.
Sharpening tip number 5:
This is the traditional sharpening step, and you will be amazed at how infrequently it needs to be done after following the previous tips.
The x-acto blade has a compound edge. There is the edge that you can see, and then another, steeper edge, that requires a jeweler's loop (or better) to see. It is fairly easy to "feel" the visible edge against the paper-on-glass. Feel the position of the visible edge, and then lift the blade up another 10-15 degrees and push the blade across the paper. Alternate sides. You should only need a half-dozen passes on each side on 1500-2000 grit sandpaper to produce a wicked edge. Finish up with a few passes on the strop.
Sharpening tip number 6:
You can adjust the angle in tip to suit your needs. If you want a sharper edge for more delicate work, use a shallower angle. For a longer-lasting edge on less-delicate work use a steeper angle.
Sharpening tip number 7:
Only perform this step after you have performed tip number 5 quite a number of times. Each time you perform that step you are increasing the size of the primary cutting edge. In this step you want to sand back the visible edge. Do about six passes on each side first with 300 grit, then 600 grit, and then finally 1500-2000 grit. For the cutting edge, do about six passes with 600 and then 6-12 with 1500-2000. Again, the grits here are only approximate and you can use what you have. Finish up on the strop.
Sharpening tip number 8:
If you have access to a jewelers loop or a small microscope (20x is fine) look at the edge while sharpening it. You will learn a *lot* about what each step is doing to the edge.
The only thing that is at all difficult about any of this is learning to hold the blade at a consistent angle in step 5.
Seriously, it takes less time to maintain the blade in your x-acto handle than it it does to replace it. And you will have a *much* better edge than a new blade at all times to boot.