Here is the ACT link to writing. If you follow the RUBRIC, you will be successful.
This is the link to the ACT site on which the essays are graded and explained.
Some of this information is a little old. However, the ACT has shifted back to the old grade standards so it is not bad.
Here is another site with some great inside hints. Please note the one on attention to actual details.
Peterson's produces the ACT materials. This is a great resource.
Great video series on the writing portion.