I'm an artist, currently attending art school
And while its not to say other people don't have glorious designs and creativity, it wouldn't sit right with me to have someone else's design on my body.
So I start a few weeks/months before hand free sketching ideas, then refining the sketches, and getting lots of opinions from my friends. then final tweaks to suit me, and then if I love it, I'll have someone draw my design on me to get a feel for the look. And then go from there ;)