Tough one - one solution that has worked quite well for us, though not with delicate archival material, is to scan or shoot the document in small sections and then stitch them all together in Photoshop. I'm sure there are other ways to do this, but this is tried-and-true.
The two main problems with this technique are a) keeping your images "square", i.e. a level straight line drawn across the entire height or width remains straight and level, and b) maintaining a fairly constant source of illumination. It's easier to do a) with a camera, but the scanner is better at b). It's a real biXch trying to correct rotation after the fact, but illumination is not so critical if your primary aim is to "clean up" the text.
If your aim is not to "reproduce" the piece, that is to make it appear just as as it is in its natural condition, but to increase the "readability" for human or computer eyes, then just two techniques in photoshop should help you substantially - the Levels control and the Unsharp Mask (USM) filter. The order of these two steps IS important, which I'll explain in a moment.
This step assumes that your scanned or digitized images contain the full scale of greys. If not, go back and make sure you're working with images whose black and white points do not extend past the white and black points you see displayed in the "histogram" in the Levels tool's display. Okay?
First, use the levels control to trim your white and black points. When I say white or black here, I mean the 100% black or white point. Set the white point so that most of the "paper" is "white", and set the black point so that just the darkest parts of the type are black". A useful technique to use here is holding the option- or control-key down while you drag the the adjustment triangles for the black- and white-point settings - you can gauge dynamically just how many pixels are turning "white" or "black".
Convert the image to grayscale. Adjust the levels once more - same technique - you're just tweaking now to adjust for conversion. Now you can use the USM filter, setting the intensity somewhere between 100-200%, with 0 threshold, and adjust the radius to somewhere between 1-5 pixels.
Your result should be fairly crisp, clear text - though this technique will also amplify any dirt in the original image. If you have a lot of time on your hands, you could try erasing all the "schmutz" with your eraser of choice.
The order is important because the unsharp mask filter works based on contrast near edges, so maximising that effect before using the USM yields the best results - for this type of sharpening.
If anyone has a better technique - I'd like to hear it too!
dmz
