This is what I use; it's the perfume- and dye-free one, of course. I only use it when I have some really stuck-on residue. A little goes a long way; I use about 1/8th of a scoop. Fill the LBK most of the way with warm water, add the Oxiclean, shake it up, and let it sit for about ten minutes. Then pour out the liquid and if there's any wort residue still in the LBK it should easily wipe away with a paper towel. Then rinse it out really, really well.
Normally, though, I just do as you do and use unscented soap. That's usually good enough.
As I normally brew right after bottling, the next step after cleaning is to go straight to adding the sanitizer into the LBK.