I've illustrated many published books and I particularly like doing the covers. So much so, that from time to time I make up a cover for self-improvement and self-promotion. Doing these "fake" covers allows me to inch out the comfort zone, since there is no expectation other than to have fun. 

George in the Gorge is my most recent sample cover. Most of it is very much like my "paying gigs", but instead of my usual big, round, cartoony clouds, I went for a painterly direction to see how it would jive with the cartoon. I think it works, but in any case it was fun. 

Since I've been working a long time (my freelance business is in its 34th year), I have plenty of published works to show prospective clients. If you are starting out, or have fewer published works in your portfolio, don't hesitate to make up a pretend job. It's fun, and  it shows the client you can visualize and execute a solution. And remember, a career in art is valuable, viable, and rewarding. Don't give up!