I strongly agree with everything Peg M. suggested, and her book recommendation "How to Talk So Kids Will Listen..." is one of the best I've read in nearly 20 years of working with young children.
Another book/website I'm also going to suggest is the Positive Discipline site. Their method is to work with children by helping them to come round to our way of thinking in a logical and respectful way. I use this with my son, asking questions to help him think about what he's doing, instead of directly engaging in correction and telling him 'no', which gets an immediate, strong reaction from him; this method helps him to figure out the "why" of what we need him to be doing. When we use this method, which allows him to come to desired conclusions on his own and then motivates him to be excited to 'do better', we often are happier with the results than just rote correction, which we all fall into from time to time. What busy parent doesn't?
I also agree that addressing the sugar issue at preschool is so important. My preschool doesn't serve sweetened snacks and parents are asked not to pack them in the child's lunches, too. When I had my last preschool, I didn't have a policy around this. Too bad, too, because in hindsight, I could set my watch as to when everything was going to begin going downhill. Ugh. Cheese and almonds, or a sandwich with protein (meat, cheese, egg) will help your daughter at the end of her day. Tired, hungry kids are often very difficult to discipline!
If you have a routine time of day for her to reconnect with you and/or your husband, that would be great too. Plan even a five-minute time just one-on-one in the morning (can your husband take the baby for 5 minutes before work?) and some alone time during the bedtime routine, plus an extra 5 or 10 minutes with one parent or the other in the evening, just to sit and play.
I also like the idea of giving positive attention to kids when they are busy at things they enjoy doing. By this, *I don't mean* walking up to the child and praising them or distracting them from what they are doing. Instead, *stand behind* the child and just stroke their hair, kiss their head, squeeze their shoulder... it's a very affirming non-verbal gesture that lets kids know that we are noticing them, even when they are just being. Works on a profoundly deep level, and do this at least 3-5 times a day if you can. (This technique is taken from JoAnn Nordling's book "Taking Charge"; another great book to recommend, and one that many preschools use with their parents in addressing behavior and discipline issues. )
Best wishes.... and remember, with these books, take what works for you, and when you try a new method to correct undesired behaviors/actions, give yourself at least ten days before deciding wholesale if something isn't working. Rarely does everything work 100% of the time, and lasting change can sometimes be slow.
I'm sure you will get plenty of other advice. Give it a shot and let us know how it goes!
H.