June 21 and 22 shouldn't be bad for lines, so I wouldn't worry about that at all. Definately get there early (8am-ish) to get your tickets if you don't have them already and be the first through the gates! It wouldn't hurt to pick up a brochure (rest areas along the highways will have tons) and study the map beforehand to know where to go. But most of all, have fun! :)
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PO!NT OF VIEW - A Different Look at Roller Coasters
http://www.crosswinds.net/~justmayntz/thrills/