If you want to know how to get flats with perfect separation, no floating pixels, and are easily selectable. This is the sum total of what you need to know:
It's all in the TOOLS.......and their SETTINGS
Set the feather to zero px, and uncheck anti-alias on the Lasso tool.
Set the tolerance to zero, and uncheck anti-alias on the Wand tool.
Set the Opacity to 100%, and set the tolerance to zero on the Paintbucket tool.
Use the pencil tool to do any fine detail color work, and make sure it's set to 100% opacity as well.
***Do not, under any circumstances, use the Brush tool for flats. Ever.***
These are your must have settings and tools, may they take you far