practice, practice.Practice.
A more efficient method of developing the muscle:Also adding weight to your pen is a good way of developing stronger muscles in your hand. Try taping used batteries to a pen and start writing with the pen. After a while the muscles in your hand will develop and you will be able to write faster when you take the batteries out in an exam.
ghabash you legendWell, I had a revelation when writing my creative. At home I managed to write 4 pages within 40 minutes with 5 to spare, when I was in the exam room I wrote 8 pages with 20 min to spare. What I learnt from that is that memorising the information you want to put to paper is important. Read through your prewritten story/essay once or twice everyday a week before the test and you'll vomit out the information quicker than you'll expect.