How to reduce edge effects in cell-based high-throughput experiments?
Lundholt et al. describe a very simple trick: let the plates settle in air at room temperature for a couple of hours before placing it in the incubator. We tried it out, and although the color scales are different in the following plots, it is easy to see that it really does help:





(Thanks to Sigrun Gustafsdottir and Kate Madden for testing this.)

I have seen some people "pad" the plate by using only the 308 interior wells and filling the  76 edge wells with only liquid, but I have no data for how well that works.

Allison Tanner from Corning has collected a compendium of techniques to reduce the occurrence of irregular patterns of cell adhesion or “edge effect” in microplates. Brifly summarized:


Avoid bubble formation in the media when seeding cells.
Use cell harvesting methods that foster adequate mixing of the cells in the media and complete dissociation of the monolayer into single cells.
Seed cells at a density that will support attachment and growth.
Allow cell attachment to occur in media of the proper formulation for the particular cell type.
Dispense cells in an appropriate volume of media.
Follow strict aseptic techniques according to respected microbiological practices.
Cultures should be routinely monitored for mycoplasmal contamination.
Minimize entrance to incubators to help reduce fluctuations in the internal environment.
Allow cells to settle in the microplates while the microplates are on the benchtop.
Culture cells in an environment that reduces the formation of static electrical charges.