My daughter is 2 1/2 and I just had a baby boy 2 months ago. I was really worried about the transition too and we do have our bad days, but for the most part, it isn't as bad as I thought it would be. I had my mom here for a month, so that helped since my daughter got one on one attention most of the time from her, but all it really did was delay reality. My daughter has still had to adjust over the past month to having someone who shares my attention. My biggest problem has been that we're potty training her too, so she uses that to get attention and it's very frustrating.
My best advice would be to try to spend as much one on one time as possible with your older child, especially in the beginning since the baby will sleep a lot of the time (hopefully). Also, have the older one "help". My daughter likes to help get baby brother dressed, get diapers for me when I change him, help with laundry, etc. Keeping him involved will work wonders.
Someone else mentioned having a present to give the older child from the baby. We did this too and it was great! I also would recommend a Parents Day Out or similar program for your son. My daughter gets to go play with her friends and she LOVES it! This also gives me time with the baby. If you can afford it, it does wonders for their social skills. We also belong to a play group and do story time at the library. Getting them out of the house sometimes helps with what seems like the boredom that sets in. I know this will all be hard at first, but you will be surprised how quickly things fall into place.
Best of luck with your delivery!