Have the child use wide-ruled paper, graph paper, or paper with raised lines to help with letter and word alignment.
Try pencil grips or other writing aids for comfort.
If possible, let them use a computer to type instead of write, and teach typing skills early.
Don't criticize sloppy work. Praise their hard work and offer positive reinforcement.
Acknowledge the condition and talk to the child about it.
Teach them ways to relieve stress before writing. For example, have them shake or rub their hands together quickly.
Let them squeeze a stress ball to improve hand-muscle strength and coordination.

Some resources and things you might consider include: