Hell, I wish I had known about this before I started mine. I researched for frickin forever online about epoxy's and other garage floor coatings. From what I've seen, 90% of it is all in the prep work.
My floor is the original from when house was built in 68, couple cracks in it, nothing major. Started off with a good powerwash, then used a etch cleaner which didn't do much so went and got muric acid and cleaned it with that which seemed to work really well.
http://www.carguygarage.com/300sqepflcok.html is the kit I got, except for 400 sq feet for my garage. I hope it lasts a good while, I wanted a bright blue floor with white and dark blue sprinkles....Guess we find out!