Illustrator is my favorite program!
The tools I use every time I open that program
1) the pen tool, it takes some getting use to but nothing beats it
2) the Blob tool, stupid name lovely intuitive tool. It has the feeling of really drawing, but in vectors.
On that front if you have an iPad get the adobe ideas app! I love it, turning my iPad into a sketch pad, with an undo key, a million color set of markers! Yes please!
3) the white arrow, to get every line just right.
I could go on and on and on, I love illustrator so!
After looking at your work I see you do a lot of things with the line tools. I would recommend expanding those line paths after you get everything just right. It's under Object>expand doing that will really help when you want to scale those objects.