I have some old documents that have been scanned, and I want to convert them to black and white. Content should be always black, and background white:
I use Photoshop.
If you have control over the scanning, or can get them rescanned, increase the contrast setting in the scan and set the black point at the darkest bit of text you can find. That would make the steps below easier. If not, read on...
Here's part of a fairly typical old document scan:
The details will be different depending on the document (this has somewhat higher contrast than your sample, for example) but the broad outline will be the same.
Notice that the yellows slider is far to the right, lightening the yellowish background. I was able to darken the text only a little.
This gets you 95% of the way there. A scanned document typically has a histogram with a large lump toward the right (the paper) and a smaller lump toward the left (text). You'll have to experiment with your documents to find the right settings.
From this point, you can duplicate the image, flatten the duplicate, and use your regular Photoshop retouching tools to clean up the remainder.
You mention Photoshop, but in case you are interested there's also a GIMP plugin that does advanced grayscale cleaning and processing:
It's called Nuvola Tools, and it's mainly focused on scanned art, but you might want to give it a try.
Source: GIMP Plugin Registry