I learned how to deal with skin colors not human:
This was the first time having to do one with hair covering it's eyes. That does not sound too hard but making it look like it does have eyes can be a challenge.
With Haithskel, my friend's avatar, I learned how to do a slightly harder pose and keep it proportional.
This one was quite a bit of fun. I had to deal with a difficult pose, lots of shadows, and making purple and yellow blend seamlessly in the eyes. I also got to work with lots of different shades of purple and making them go together, which was a fun process.
WIth this one I learned how to deal with technology fails. SAI crashed and all I had was the png version, which lacked layers. I had nearly completed the piece when it crashed and I had to figure out a way to deal with the lack of layers and completion of the shirt. After four hours this is about all I got. I only wished I could have done better shading.
With this one I learned perspective a bit. I had a bunch of fun with the blending of the shirt.
With my latest one, I learned how to do two tone hair, as well as working with a sitting version in SAI. I found it much easier actually to deal with sitting on SAI than by hand as I had with a previous chibi.