If you want to try several different ways of scanning a negative or slide, you don't need to physically scan it more than once. The raw scan data for both previews and scans are kept in memory, and can be reprocessed using the "Save" button.
The default options have been designed to work well on most images and most systems. If you're having problems scanning, reset all the options to their default values by choosing the "File|Default options" command. If this works, try changing one option at a time and re-doing the scan.
Each time an image is saved as JPEG, some information about the image is lost, even at the highest quality (and lowest compression) levels. This effect is compounded if you save a file, then make some changes and save again. So unless you are certain you will never need to modify an image, JPEG is an undesirable choice.
Instead, use TIFF, which always stores all of the data of an image without any loss, even when compressed. TIFF is therefore a good choice for archiving of files, and for subsequent image operations in tools like Photoshop(TM). The downside of TIFF is that the files are larger than JPEG, even when compressed, and especially at high resolution offered by today's scanners, this can result in very large files.
Consider burning TIFF files to CD-R files for permanent archival.
Much of the time needed to perform a scan is in moving the scan head (for flatbed scanners) or the media holder (for film scanners).
These two tips will help reduce scan times for flatbed scanning.
The scanner will only scan the area specified in "Crop|Preview area". If you are scanning a single photo or strip of film scanner, adjust the cropping around the preview area, set this option to "Crop box" and press the Preview button.
Also, orient media in landscape on the scanner bed. The stepper motor that moves the scanner head is usually the limiting factor; the less it needs to move, the more quickly the scan will complete.
To get the cleanest scans of characters when scanning printed documents on a flatbed scanner, set "Input|Media type" to "Text". This will ensure the result is black and white, which will reduce noise.
Typically OCR packages expect TIFF file format; set "Output|Output TIFF file".
You may also find the "Output|TIFF multi page" option useful, as multiple pages will then be saved in a single file. Note that you need to remove the "+" from the "Output|TIFF file name" option to use this.
If you save images that are significantly larger than the amount of memory on your computer, set the rotation to None. This will make the cropping go much faster.