What are your recommendations for cotransfecting several plasmids?

The recommended amount of DNA being transfected should be split between the different plasmids. For example, if you normally transfect 5 µg of one plasmid, use 2.5 µg each for two plasmids (other proportions of the two plasmids can be used, as long as 5 µg total is maintained) as a starting point for optimization.