ATI seems to have a good name....
A great way to learn is to do it awhile, then take a site visit to a place like Jepp or Universal and sit down with the planners while they plan a few trips. You can learn alot looking over their shoulder as they plan a 50 flights a day and process the changes that happen....Its very fast paced and they are very efficient and teach you how to process slots in real time, see the overflight process. Well worth it.