We've been struggling with DS1, who is 33 months, too. I have been trying to implement the advice from the books Siblings without Rivalry, Raising our Children, Raising Ourselves and Discipline (Brazelton). He has been having a lot of tantrums and defiant behavior. He has been hitting. He has been pushing buttons purposefully, pushing limits, etc. He is also usually sweet with the baby.
The advice that I have found helpful is the following:
Validate feelings - DS1 has said on numerous occasions, "I don't want you to hold the baby". Instead of responding, I hold you too, or I have to hold the baby, or other defenses, simply say, you're upset that I have to hold the baby. It makes you feel sad to see the baby with me so often...etc., then follow up with, but I like to spend time with you too, I enjoy all the fun things we can do together. But, most importantly, validate their emotions.
Do give them some special, one-on-one time (only 15-30 mins day but just you) - your sweet little daughter is still in there, and she will come out to play with you. Come up with a new, constructive activity to do together...
Consider their pushing buttons as really just testing limits - they need to know that you will react consistently but not angrily, that there is order and discipline in their lives.I Try not to get angry (I know, hard with all the hormones running around) and respond firmly and consistently.
Don't feel bad about using crutches! Okay, this is not from any book, but putting on more shows and offering special treats is okay when you have a new sibling in the house! You can decrease their use later on.
So far, I have benefited quite a bit from the validate point -- it really seems to work for us. And, since I have been healed enough to do things with DS1 one -on-one, things have really improved. Also, I have to make a concerted effort to be cheerful sometimes, but it pays off in that he's in better spirits too. It's really hard to keep in mind the pushing buttons as just testing limits and not getting angry! That has been hard to implement!!
Anyway, I really look forward to hearing others' responses!!!