Especially from unsuccessful design at work that make you to sweat and spit blood
At school you learn nothing, application notes from ICs manufacturers are good knowledge source also
a combination of daily work, paper and book reading, less university courses (mostly only the very basic, however it is very important to know this), and of course colleges. the most i learnt of false working and defect circuits.
The base knowledge from studies is always helpful. Every new design at work teaches a lot. Self mottivation is the best source of knowledge. Internet revolutionalized the way we learn.