I'm due in July and have 2.5 yrs old and a 1.5 yrs old. When it comes time for you to head to the hospital your older one isn't going to realize its any different then when you've left the house without her before. We've gotten a 18" doll for my older one to take care of when the baby comes home. Also making it really great to be a big sister helps - pick out the baby clothes, grab diapers, etc. The most important thing though is to make sure you get 1-1 time with your older one. Set aside time (probably when the baby is sleeping) to have time just with her doing something that she loves - dress up, reading, puzzles, etc. Just remember woman have been successfully bringing home new babies since the beginning of time; you've got this! :-)
She's young enough not to really notice a lot. I'm due June 24 with a girl and my boy will be 2 July 11.