You could read through a few tutorials on how to get the best from scanning. These links will take you to two of many across the web.
http://www.jimsdigitaldiary.com/scanning-tutorial.html
http://genealogy.about.com/od/digita...can_slides.htm
Like everything else in life, the best results always come out of the amount of effort you put in.
EDIT. Do remember that you will get tons of dirt and scratches show up that will need cloning out. You will also get artifacts that will look like blobs of dirt. If you get a clean scan first off you will be very fortunate. You have to factor all this in when you start scanning as well.