It's hard and the house is a mess. I have a just turned 3 year old and a 5 month old. Things I've found that help are accepting that beans on toast is an adequate dinner for DD1 (and me!); preparing snack-type meals that can be done in advance e.g. raw veg sticks, sliced ham, bread/sandwiches, so that I can just pull it out of the fridge when I need it; cooking and freezing meals to reheat in microwave; minimising housework (e.g. I never iron anything ever). I try to get out at least once a day, even if it's just a walk in the rain 'to splash in puddles'. In theory getting my changing bag packed the night before is best, but in practice it rarely happens - although when I manage it, it does make the morning easier. I have friends over with kids and ask them to stay for tea - entertainment for DD1 and another adult around to hold the baby/serve food etc.
One bit of advice I was given was to describe what DC1 is doing when you're busy with DC2. So when I was changing DC2's nappy or feeding her, if I wasn't actively having a conversation with DD1 I would say out loud what she was doing e.g. "DD1 is stacking some blocks. First she's picked up a blue block. Now she's doing some red blocks". It does make you sound absolutely barking mad, but the theory is that DC1 knows that you are noticing them.
I also have a stash of chocolate that I sneak when DD1 isn't looking . On a bad day I start it in the morning, on a good day I make it through till tea-time.
Lifeguard I like that idea of telling the baby to wait - DD1 is always hearing 'just a minute DD1", I'm sure she'd like to hear it the other way round sometimes.
Also <waves> to Babycakes - we were in the same Due Date club with our firstborns
Oh just remembered - I have a running 'to do' list on my phone, with things like 'label DD1s clothes for nursery". It's a long list and progress is very slow. I have recently inserted "Get through day" into the list. Every time I read the list now it makes me smile and feels like I am accomplishing something even if it's just to survive!