hey Abby,
you got lots of great responses. I feel for you. My son was 2.5 yrs when my daughter was born 7 months ago (i.e. still going thru it).
Here are some ideas that helped us...
don't leave them alone together so the baby doesn't get hurt.
MAKE alone time with the toddler -- shoot for 30 mins a day -- maybe during baby naps. Focus on the toddler during that time.
have the toddler do things to help with the baby like fetching diapers, patting her while you hold the baby to burp, etc. Praise her like crazy when she helps out. Praise her for any nice/good/helpful thing she does to build up happy feelings against what anger might be there for what i suspect she often hears "DON'T...the baby!!" I got really fed up with my son and i had to try hard to be positive with him instead of reprimand him constantly. i've read it takes like 5 compliments to negate an insult for adults.
tell her the baby is her baby and you need help taking care of it. of course it's your baby and she has to do what you say, but over time comments like "we don't want to hurt your baby, do we? Let's be gentle" will take effect. it seemed like when i gave my son some ownership of his sister, he was more careful with her and became protective of her.
Do things with the two of of them. For example, you can't not feed the baby. So have the toddler help you bottlefeed or have her sit with you and cuddle while you breastfeed. Include your toddler in the activities you have to do with the baby. Right now the baby won't notice, but the toddler will remember feeling left out or resentful of your love and attention to the baby.
Tell the toddler how lucky she is to be a big girl. for example, her poor baby can only have milk and that's boring. The lucky big kid can eat (name her fave foods). My son didn't care at first. But as months have passed and i'm strict on the DO NOT FEED YOUR BABY SISTER BECAUSE SHE HAS NO TEETH AND WILL CHOKE he realizes it is true. Now he talks about how how he's big and can have whatever he wants while sister is just a baby and can ONLY have milk. it makes him feel special.
get the toddler involved in play with the baby as much as you can -- be creative :-) Now that my baby is more active i lift her up and down while my son jumps on the bed. it's like they are bouncing together and they both laugh uproariously together. My partner holds the baby and carefully chases our son around the house with her so it's like they are playing tag. Again both kids screech with laughter. My son warms up to his sister much more when they play together.
This is still hard for me to master... I've read that you shouldn't say DON'T to your toddler all the time. Instead of, "Don't poke the baby!" say, "That hurts the baby. Let's think of a way to play nicely with the baby."
Teach your child how you want her to respond to the baby. I've learned in these exhausting, frazzling recent months that they follow your lead. If you get angry and yell when your toddler and/or baby cry, the toddler will start yelling at the baby when it cries. If you hold the rage inside, take a deep breath, find a wellspring of compassion inside, and respond patiently and gently to both children, your toddler will respond to the baby the way you do. My son coos, "It's O., it's O." to his sister as she wails -- my mantra to keep insanity at bay.
Similarly, now that the baby can crawl and steal her brother's toys, I've shown him to go find a toy he can part with and give it to the baby to play with so she'll leave his stuff alone. The spine chilling, window rattling, enraged shrieks of, NOOOOOOOOOOOOOOOOO! That's MIIIIIIIIINE have decreased a full 75%.
It's O. to tell the toddler how you feel. i was surprised at how well my son responded when i felt upset and said, "I really don't like it when sister cries and i don't know how to help her. It makes me feel mad."
in my opinion, timeouts have limited worth at this early age. when my son has committed a capital offence like injury or destruction i say in my calmest, biggest most serious mommy voice with the narrowed down eyes, "YOU ARE GOING INTO TIMEOUT FOR TWO MINUTES." That's when he knows he's in serious trouble and timeout is more for me to regain my composure than for him to 'think about what he's done.' after 2 mins i explain why he was in time out and why what he did was unacceptable. then i hug him, kiss him, have him say sorry, and we go about our day.
be aware that it goes back and forth. Sometimes my son is a stellar example of everything a big brother should be. Other times he's a little monster to his sister and he drives me crazy because he was just being so good to her just last week. When things shift, I look to myself and try to find a cause (e.g., spending less time with him makes him act out more).
Finally, I think things like timeouts and taking toys away are punishments and in this situation that's of limited use. Your toddler shouldn't be punished for resenting a new baby. She needs to be encouraged to take part in the new baby's life and to be reassured that she's loved as much as ever and that the baby is just an addition to all the love in the family.
Sorry this is so long. It's all I've got :-) I'm still learning and I truly hope you can find one or two things that will help you out.
Best wishes and good luck!