Hi, I have two recommendations
1. Don't worry about how neat your handwriting is, just get it on the page ( Though, try and make it legible enough to read)
After the exam, the neatness of your handwriting will not be your biggest concern, all you'll worry about is your mark, so just make sure that whatever you've got to say, say it. However bear in mind that if markers cannot read your handwriting you'll risk losing marks, but if it's legible enough to read you should be ok.
2. Strengthen your understanding of the topic this will give you more to write about and strengthen your response in answering the given question. ( Bear in mind, make sure that it is relevant and helps support whatever you are trying to say) Get as much as you can from classwork and other resources, relevant to your subject.
3. Hold your pen lightly don't press down so hard, just hold it and write. Also, it may help to have those lightweight gel quick-dry pens.
4. Look at past band 6/ Band 4 responses, this will give you a better idea of how you could improve your own writing and things to talk about if you are running out of ideas. Ensure that is done beforehand.
Hope this helps! (: