These samples will help you to understand the concept of designing based on inspiration from nature.
30/60 projection
tutorial for 45/45 perspective projection
.
45/45 degree perspective
45/45 perspective projection
Taken from Presentation Techniques,Written by renowned designer Dick Powell
Lines and basic exercises
This is going to help you control and command your lines
Sketch Aerobics in Puerto Rico
sketchatoy on Vimeo. This video was taken from the sketchatoy.blogspot.com