From Dean Simonton, via Irfan Alvi's review of his book. The more surprising ideas are that:
1) "In some domains, overtraining can be detrimental to creativity. In these cases, cross-training can be more effective." Presumably cross-training involves learning about a variety of different topics. This seems useful either because it allows you to apply ideas in other fields to your own, or because it allows you time away from your main field to avoid getting bogged down in details. Or both.
2) "The creative processes underlying genius-level achievement are still not well understood, although use of heuristics and combinatorial thinking appear to be typically involved." It'd be nice to know precisely which heuristics lead to genius level output!
3) "Psychopathology has a positive correlation with level of genius, but outright madness inhibits genius and higher intelligence tends to provide the capacity to prevent outright madness." This jives with some research. For example, this study considers the effect of the interaction between cognitive ability and neuroticism on managerial performance. Regressing cognition and neuroticism alone explained 4% of the variance in performance, but adding the interaction term raised this to 19%. And after partitioning their sample in half by cognitive ability, they found the higher half had a positive and significant relationship between anxiety and performance, while the lower half had a negative and barely non-significant relationship.
I am still heartily recommending Simonton's Creativity in Science to people interested in these issues.