Even cell distribution in the 6-well plate instead of a ring of cells around the edges?

Performing cell culture experiments in round well plates can often lead to troubling spatial dispersion of the cells.  This can be attributed to culture incubators having high vibrations, non-level incubators or bad practices such as swirling the plate after adding cells.  In order to achieve even distribution of cells in the wells of culture plates, limit vibrations in the incubator, make sure incubator is level and use gentle pipetting to mix cells with reagents.