Why is my crochet chronically curled?

I have had this problem in the past as well. I find that controlling the tension solves most of the problem. It is important that your stitches stay the same size throughout the project. I usually use the head of the hook as a guide, making sure the loop glides easily over the hook when pulling the yarn through. Also, since you are using single crochet, make sure you chain 1 at the end of each row before turning. Sometimes, it can be easy to place 2 sc in a few stitches without realizing it. Try taking the time to count your stitches each row until you get the hang of it. If you start with 31 chains and sc in the 2 ch from the hook, you should have 30 sc stitches at the end of each row. Also, I have found that sc is often such a tight weave that it doesn't like to lay flat until it has been washed or pieced together. Good luck! Hope this helps.