Cleaning a starter container: Stainless steel scouring pad!
This may be known by the old-timers, but I've found a way to get the insides of my starter containers really clean.
It seems that each time you feed a starter, you really need to move a portion of the existing starter to a new container. This leaves a small amount of "smudges" of starter on the sides and bottom of the old container - and somewhat of a PITA to get out.
I've found that using a stainless steel scouring pad does an AMAZING job of getting all the residual starter out - and the magic is that it does NOT clog up the pad. I think it must break the starter into such fine parts that they rinse away with the cold water. I've done this on plastic ZipLoc Twist-n-Lock containers and other plastic and it does NOT scratch or damage the plastic. You just need to lightly twirl the pad against the sides and bottom - and VOLIA! They come out clean! :-)