I thought the shakes would help the rossettas! *:D
Seriously though, I have found that it has taken me 3 years of barista work to truly control the pour to get that lovely thin pour needed to put the finishing touches on a rosetta.
I find that gripping the jug in an open hand wrapped all around instead of clasping the handle can add some steadyness. I will post pictures if you are unsure.
Ive had after dinner coffee at our cafe after my 25th for the 20 of us and the art i poured was above standard. Maybe try evening out the caffeine with a tipple or two.