The most important rule is that the below points are suggestions only. Don't take them as THE truth, but take whatever you find is useful and discard the rest.. ok?

1. Use the Best Source
Scan of the highest quality source you can find.  An example hierarchy is film/original artwork > photos > prints > 'art' magazine > magazines  > newspaper. The poorer the source the more problems you are going to have. You can do amazing repairs to an image but the cost is time and the result is never as good as a scan from a better source. The third scanning project looks at dealing with poor quality sources.

2. How much is Enough?
For a colour image at a one to one ratio i.e. the digital file will print at the same size as the original, you do not need to scan at higher than 300ppi. For example, an 8x10" print scanned at 300ppi yields a file size of 20meg! That's a lot of information, enough to produce a photo quality dye sub print, or be run through a four colour press at 150lpi with great results

If you must enlarge at the time of scanning you will need a scanner that has a higher optical scanning resolution so that when you scan at 600ppi to enlarge the file you are still getting true information.

If the image is for the web then you will have to decide whether to scan at 72ppi initially or scan at a higher resolution, make all of your corrections and processing then sample the image down at the end. There is no real agreement on which way is best - experiment and suit yourself! Scanners don't perform at their best at low resolutions...

3. Dealing with Line Art
The theory for Line Art is that for optimum results the resolution of the scan should match the resolution of the output device. There are no extra tones in Line Art to smooth the edges so this theory makes sense. But in the real world the eye will not see any improvement at resolutions above 600ppi. This is due largely to the imperfect nature of the printing process.

4. Maximum Colour Information
Most scanning utilities allow you to scan in Bitmap, Grayscale or RGB mode. It is always better, if file size is not an issue, to capture as much tonal information as possible and discard as necessary later. This means scan grayscale images in RGB, Line Art in Grayscale.

A good example of the benefits of this approach is that if you scan a grayscale image in RGB you then have the options of repairing the individual channel problems that your scan may have. If  you had scanned in grayscale, all your problems would be merged into one layer - much harder to fix.

Some scanning utilities now offer a direct conversion into indexed colour mode. Please don't do it! This is an area where you need fine control and to be able to see what exactly is going on. Scan in RGB and do the conversion in photoshop.

5. Crop Now!
Do not scan information that you do not need - crop of the excess now. The reasoning here is file size management. If you only intend to crop out a section of an image why carry that extra file size into Photoshop and slow the whole process down. Nuke it now.

6. Use the Preview
Preview the scan and really pay attention to it! It's too easy to miss problems that could cause major drama at a later stage! Before you do the final scan, re-check the tonal range, the scanning resolution and the file size.