I always think out my sentences before I begin to write. This way, I don't stop in mid-flow causing two different handwriting efforts in one sentence. That makes a sloppy, confused look in your writing.
I always write a few sentences on another piece of paper before beginning in earnest. This helps me get my hand in position and ready for the task.
I use pens with smooth rolling ball points or fine point felt tip pens with steady color flow. Pens that skip or have sharp points on the writing surface are difficult to control and cause sloppy bumps and jags in the writing flow.
I put another paper or two under the one that I am writing on to create a padded surface. This helps the pen move more smoothly across the paper.
I use my left hand to hold the paper steady while I write with my right hand.
I try to think in straight lines so that I can imagine where the letters should go.
It helps me to put a piece of lined paper under an unlined paper to give me some guidelines.
It helps to write a first draft if the paper is very important and you only have one shot at it -- such as writing on a greeting card, filling out an application, or when you are given a special form for your answer.
Check the spellings of your words before you write. This saves erasing, scribbling out words, or using White Out to fix unnecessary errors.
Try to remember to lean all of your characters in the same direction.
Form your letters correctly with special attention to proportion.
Practice writing. It is fun to write poems or limericks that no one will see. When you practice, do your best. Whatever it is that you do most often will become what you do when you are in a hurry. If you always strive to use good penmanship, even when you are writing something that others will not see, you will find yourself writing well when you are in a hurry.
As Pat Parelli says, "Practice doesn't make perfect. Perfect practice makes perfect."
And remember, what you write today may become a document of historic significance in the future. Examples: Anne Frank's diary, CS Lewis's letters to a friend in America, Grace Bedell (young Lincoln supporter)’s letter to Abraham Lincoln during his campaign telling him he would look better with a beard. http://members.aol.com/RVSNorton1/Lincoln50.html