The basic one is to remember the universal law that everything you send out will come back to you. This happens as small as having a bad day, having bad feelings, dwelling on those feelings, and your day just gets worse, or smiling at strangers and watching them return it.. or as large as casting a harmful spell on another person and years later finding yourself greatly harmed or in another way made completely miserable. Once I cast a good luck spell on a friend. I had good luck for a few months.
You cannot change another's will. This does go back to Karma. Think about how you would feel if someone did that to you? Something in life would likely come to you that gives you those negative feelings. Not only is it not a good idea, and ethically wrong, but its nearly impossible.
Basically just think long and hard before you cast anything. Think of how you word things. Think of your emotions at the time you plan to project energy. Make sure everything is in check and you are satisfied with any possible repercussions.