This is how I do it:
- I scan the page in 300dpi
- I use the measure tool in Photoshop to get the rotation angle (and be sure not to be in indexed color mode)
- I rotate the image with the given angle.
- Crop the image
- Apply various color correction / levels. Depends on the scanner. My scanner gives images with too little contrast and colors. So I use levels to increase the contrast, and then increase the saturation.
- I resize it to 150 dpi
- Last I apply unsharp mask with amount: 80% and radius: 1.
I've made a macro with steps 5-7.