Tip 1: If you need to scan an image, and you want to use it at close to its original size and you are NOT going to print it, scan it at approx. 85 dpi. This gives you plenty of pixels to play around with before porting the image to the web, iMovie, etc. Basically, when the image is only going to be displayed on a video screen only, all you're trying to do is to get the right amount of pixels in both height and width in order to produce the physical dimensions you want. That's all there is to it.
Of course, tons-o-people will tell you to scan at a mere 72 instead. My advice is 85 because I like having a few extra pixels up my sleeve. However, in the words of the great rock band Blind Faith... Do What You Like.
Tip 2: If you are going to print the image, here are a few basic guidelines:
If you don't see an enlargement option, increase the scanning resolution (dpi) instead. To enlarge a 2 x 3 inch photo to 5 x 7, you would increase the scanning resolution by the percentage of enlargement. For example: dividing 5 (desired width) by 2 (actual width) yields 2.5. This means you need to enlarge the photo by 2.5 times, or 250%. If you want the image to be 300 dpi in the end, multiply 2.5 x 300 which yields 750 dpi. That's your scanning resolution for a 250% enlargement.
Tip 4: When scanning something from a magazine, look for an option in the scanning software interface called Descreen. This will decrease the funky, slightly circle-shaped pattern you can get (also called a Moire--pronounced Mo-ray). If that doesn't do the trick, then in Photoshop (or Elements) try applying a small amount of Guassian Blur to the image, then follow it up with an Unsharp Mask.
Tip 5: Be sure to check out the step by step tutorial for straightening and cleaning up black and white document scans. Though the examples are shown in Photoshop Elements, the steps are easily followed in Photoshop.