Remember, the 3 Ps! Pencil, Paper, and Posture!
1. Pencil - make sure your pencil is pared (if you are using a pen, don't use an inky pen that will show through the other side of the page!)
2. Paper - the page should be slightly slanted and the hand you don't write with should hold the page/paper firmly on the table to stop it from moving
3. Posture - sitting up properly with feet flat on the ground
Some other reminders: (source: http://kidshealth.org/kid/grow/school_stuff/handwriting.html#)
Let the lines guide you!
Lined paper is your friend! Those lines can help you create letters that are the right size and proportion. Proportion means that one thing is the right size compared with the other. So your lowercase "a" should be half the height of a capital "A.".
If your writing is hard to read or you erase a lot, try slowing down a little. Sometimes, going slower solves the problem. If you rush, it's hard to control where you stop and start your letters, and you end up making more mistakes.