Great advice from the great Jillian Tamaki, and it applies to more than just illustration:
I know you’ve answered this question like a bagillion times but I thought I’d throw it out here anyway, since I can: any advice to a excited/somewhat terrified 21 year old artist who is working towards having an illustration career some time in the not-so-distant future?
Hm. I teach a lot of very freaked out 21-year-olds. I think a lot of people psych themselves out. “What do people want?” “Will I get a job/jobs when I graduate?” “What is illustration anyway?” Those questions are hard to avoid and I certainly struggled with them myself. However, my piece of advice is to try not to think so “large”. Think small. Think about the marks you want to make on the paper in front of you… the ones that bring you pleasure and satisfaction. You can’t control what other people think or if they’ll give you a job. You can only control your own actions and the work you produce. You have to be a little delusional to pursue a life in the arts, so throw caution to the wind and make pictures that excite you and hopefully the world will agree.
I actually don’t agree completely—I think creative life in the year 2012 requires that you think both large and small at the same time—but really, “think about the marks you want to make on the paper [or the screen] in front of you” is still the kernel. That’s worth painting on your wall. That’s where everything begins.