Reading other peoples' code.
PRACTICING coding - This is different from pet projects... This means I learn a specific algorithm or data structure or other coding construct and then I continually build it over and over from scratch every night until by the end of the week I can do it in 5-10 mins blindfolded. This is very powerful... Coding can become "muscle memory" just like anything else. This is how I can excel in interviews... Where I am often nervous and can't think straight... That's ok if I don't have to think anymore :)
Filming videos/teaching others/speaking. Just like j stated... Teaching others, due to my extreme desire to not spread incorrect information, forces me to check my own knowledge and also I've learned that being able to verbalize what I know is often as important as what I know and often times I cannot even do it properly... So this helps me with that skill as well.