Be aware of how hard the first couple of days are when all you have is the tiny amounts of colostrum - on the second night, I was up with DD3 almost every hour and the emergency pack of Aptamil looked SOOO tempting - but then the milk came in and she settled to every couple of hours, we have even had a couple of 3 hour stretches at night when she's had a good feed (I try to tickle her hands or feet to keep ehr going to take plenty!) She's 2 weeks old now and, touch wood, things are going better than before (DD1 tube fed in incubator then bottle fed, DD2 breastfed but we gave her the odd bottle cos I caved in)
If your 21mth old naps still, and the new one goes to sleep, grab the chance for a few minutes yourself! I read somewhere (not specifically about babies) that 20mins is the optimum "power nap" - less isn't enough, more and you can wake up groggy.
Don't plan to do too much in the first weeks - I have a "to do" list (I'm anal like that) but now I just mark 3 things that I have to do - anything else can be forgotten! Today it was change the bedding, charge the camera batteries, do the bill payments online - they've been done, so if I get anything else done on the list that's great, but even if I don't I can feel proud that I've achieved my targets!
Drink plenty of water and make sure you eat regular meals plus snacks. If you have to make packed lunches, do one for yourself as it's Sod's law that when you get the bread out to make your sandwich the baby will wake up and you'll reach for the nearest crisps/biscuits instead.
Think of the effect on your figure! BF is great for losing baby weight initially!
Read all you can and look at pics about proper latching on - it is the crucial difference - I thought i had it right last time but I hadn't as I was quite sore - I looked at magazine articles/NCT leaflets this time and had it fully in my mind as to what to look for.
If you can, put the baby to your breast as soon as poss after the birth. With DD3, after the cord had stopped pulsating and been cut, the midwife gave her a quick rub down and gave her to me - I put her to my breast and she started to feed after a few minutes (ooh, I'm getting all emotional now! It was lovely, MW dimmed the lights as soon as the placenta was delivered and checked and she left us to it while she made tea and toast!)
Another thread had a link to this website with good videos (apologies if someone else has already said this and I've missed it): Dr Newman video clips of feeding