I'm on my 3rd time around... and there are a few things I've learned since my 2nd as well.
First of all, the single best decision I made with my second (that I didn't do with my first) was to buy a wrap and start baby wearing. This saved my sanity on hundreds of occasions (gave me my hands free to help my DS with things, and DD was super fussy and clingy and I could carry her around while cooking/multitasking). A huge life saver.
With my first, all the doctors, midwives, OBGYN's, etc, swore he was going to be a BIG baby ("at LEAST 10 lbs") so I didn't buy any newborn clothing. Ultimately they were all wrong and he was a perfectly normal 8lbs 2oz and all of the 0 - 3 month sizes swamped him, and DH made a mad dash to the store when DS was a week old to buy some more clothes as the pants/trousers kept falling off of him (and he was in the newborn-sized diapers for the first 3 - 4 weeks). With my second I was better prepared for any size (and she was small, and stayed in the newborn sizes for more than a month).
With my second, I was way better prepared for breastfeeding. I had loads of issues with my first but with my second I had my pump cleaned/sterilized and ready for use (if needed) and brought it to the hospital with me as well.
I also went to the hospital prepared for a multi-day stay. I'd had an EMCS with my first and while I was hoping for a VBAC, I had enough clothing for me and DD in case we had to stay longer (we did).
With my first, I made several homemade meals in advance and froze them in preparation, but found it to be really unnecessary as I had plenty of time to cook once I was back on my feet. So with my second, I didn't bother with this precaution--but afterwards I wished I had! I found life with 2 to be REALLY hard in the beginning, and I really struggled and wished I had thought to prepare some meals in advance. With #3, I will be doing so this time again...
Some people adjust to a second baby with little-to-no problem, but I really struggled (and suffered with PND for 6 months) which I was not at all prepared for. I think if I had mentally prepared myself for the difficulty I might have been able to cope better. I definitely feel better prepared for #3...
And lastly, while I was on maternity leave with #2, I took my DS out of daycare so he could be home with me. In retrospect, I should have kept him there (he 'only' goes 3 days a week) as it would have been easier on the both of us. He's in school now though, but both he and DD will continue to go to our child minder this time around so I can have some quality time with the baby and catch up on sleep when the baby naps, etc. I was a walking zombie for months and months the last time around (as my DS wasn't napping by that time anymore).