I really like the book The Happiest Toddler on the Block for dealing with toddlers and tantrums. It talks about how to short-circuit a tantrum by connecting with your child and respecting his or her feelings.
It's awkward to do in public because the book advises talking to your child in "toddlerese" as you reflect back her feelings. "You want a cookie! You really want a cookie! You say cookie now!" But I found that it was pretty effective once my child was starting to become verbal.