As previously stated the sites listed at Beachside are awesome but even with early bookings are hard to get because of their popularity.
Get the Oregon Coast Mile by Mile book and check out the areas that you might be most interested in. The Oregon state park reservation system works well and you can see actual pictures and descriptions of most sites at any particular park. We have been to the Oregon coast 3 different times and if you would like some more particular info please feel free to PM me.