DD1 was relatively easy, so I thought no 2 would just slot into existing routine. I therefore got a bit of a shock when DD2 arrived. As there's a 3 year gap between them I'd forgotten those first few weeks...
For me, the first 8 weeks were terrible (mainly because DD2 screamed so much until we had cranial osteo sessions, which sorted her out, but that's another story).
Things I'd now recommend:
If you can afford it, or have willing family / friends, get help with cleaning and ironing for the first month. I have a fab friend who would come round, make me tea and then do the hoovering whilst we talked! Most people tend to feel a bit more positive if their surroundings are not out of control too..
On the same note, a small point, but an important one: make a stock of treats for yourself, and pop them away in a cupboard - a magazine, a scented candle, some wonderful shower gel or a new pair of slippers - that way you can treat yourself to something when you feel low or need a physical boost.
Arrange playdates for the toddler, but get them used to this now, if they've not done it yet, so that they don't feel excluded when the baby arrives. Again, I have a couple of friends who regularly took the older one off to burn off energy in the park - essential in the winter months when you won't feel like standing in the cold with the baby in the pram (not to mention trying to b/f them there if you're intending to!)
Get lots of suitable videos / CBeebies on hand for feeding time. Alternatively, put together a bag of special goodies (books, new toys, sticker pads etc.) that only comes out at this time. (However, I found it tricky turning pages whilst b/f).
Get your shopping online (Sainsbury's currently delivers free on Tues, Wed, Thurs for orders over about £70.)
See if you can get help occasionally for the 4-7pm period to cover tea, bath and bedtime. This was the time I found the hardest, when the baby cried and needing feeding lots, but the toddler was also knackered, yet needing to maintain her regular routine. Do you have any willing reliable year 10/11/12/13 girls nearby, who might appreciate a bit of extra cash, or are doing their Duke of Edinburgh award (does this qualify for the community service bit?!). I found an extra pair of hands, plus another adult to talk to, made all the difference.
Prepare and freeze as many meals as you can in advance, so that supper is there, quick and healthy. My dad and MIL did this for me, being unable to help in other ways. Friends also popped over with the odd cassarole (better than yet more baby toys!)
Don't expect much of yourself or the children for the first 8 weeks. Clear your diary, let friends and family know that you will be taking things as they come, and not to worry if you disappear off the planet for a while.
Don't expect the toddler to understand the need to keep quiet. I remember being suddenly amazed at how loud DD1 is - you interpret the behaviour of the older one through the baby's eyes and the temptation is to see the toddler as much more capable than they are. I have to remind myself that DD1 is still only 3.5.
Remember that babies' routines change, so just when you think you've got it sorted, the bathtime / bedtime routine will change, again! Initially I felt so torn between the two of them - I felt like I was failing both, but gradually, as DD2 has become more of a little person, and less of a blob, things have got much easier. By about 4/5 months, things had definitely got to the stage where tea / bath and bed were not stressful any more.
Hang on in there - it's wonderful when the younger one starts to recognise their older brother / sister as much as you. We now feel as though we are a family, rather than a couple with a child.