Simply slapping your picture on the glass and pressing "go" isn't going to make your precious photos shine. It's important that you experiment with your settings to see how adjusting contrast, brightness and sharpening will affect the overall look of your pictures.
Most scanners come with automatic settings, which are a nice shortcut. These settings allow the picture to be scanned at the highest resolution. Of course, the highest resolution might not be necessary, so customizing the settings becomes more important. You'll want a smaller file size (and thus lower resolution) for images that are going online or are e-mailed. If your goal is just to preserve your photos, a minimum of 300 dpi should be fine. You can get away with as little as 200 dpi if you're just going to post the images on the Web or e-mail them. If your final picture is going to be bigger than the original or you need a higher resolution to create a more professional looking product, go for 600 or higher dpi.
Choose your color. If the photo is in black and white, you can either scan it in grayscale or color. The color scan option will give you a greater ability to manipulate the image. You can change it back to grayscale once it's been scanned. If it's sepia-toned, scan it in color. Oftentimes, you can also find specific document types in scanners that will choose appropriate settings for the image type. Newspaper pictures or articles, illustrations and even negatives have presets in certain scanners.
Don't forget to use the "preview" or "prescan" function after you're done adjusting. This is an invaluable tool for doing a final pass of what the document looks like before you hit go.