Would you be interested in a great book? It's called the Power of Validation.

http://www.amazon.com/The-Power-Vali.../dp/1608820335

I have a pretty smart, sassy girl as well. I know how it can be. You have to be firm and learn to ride through the tantrums. Spanking is never a good option.

Parenting a Child Who Has Intense Emotions: Dialectical Behavior Therapy Skills to Help Your Child Regulate Emotional Outbursts and Aggressive Behaviors: Pat Harvey ACSW LCSW-C, Jeanine Penzo LICSW: 9781572246492: Amazon.com: Books

There's tons of reading on tantrums. Don't despair, you are doing great and you are very loving. Some kids are tougher cookies than others. You may also want to consider play therapy, I got it through city programs and even paid out of pocket. It was so helpful to have another adult help me and validate me when I thought I was a bad mom.