The best tip I ever got about illustration was to study Dr. Edwards' book.
http://www.amazon.com/Drawing-Right-.../dp/1585429201
Here are samples of her student's before and after drawings that is part of her course. Dr. Edwards has you draw self portraits once before the actual course as a test to see where you're at and once again at the end to see how far you've developed. The improvement is almost always staggering.
I worked as a traditional animator for a few years and every supervisor or colleague that I worked with swore by this book. I didn't read it myself until I was already working in the industry and it still had a profound affect/improvement on my art. Dr. Edwards makes drawing as easy as writing a paragraph and she can literally teach anyone to do it.
If you haven't already checked out this book, I can't recommend it enough. Once that kind of proficiency is under your belt, your creative expression with vastly expand and even if your art doesn't change that much, it will become easier to crank out.