I know that in part, I am fortunate that my son is pretty crazy about his little sister. He'll be 3 in January, she is 7 weeks. I SAH but we are very far from family, and our friends, of course, work. We can't afford sitters or any kind of "preschool" atm, so it's just me till about 6 each day.
For sure it's hard, I think what helps me is reasonable distractions for my son, and things in my own reaction that seem little.
His good distraction is a wooden train set that was a "gift" from DD when we got home from the hospital.
But I think the bigger thing is trying to keep the mentality of mostly tending to DD's basic needs-food, clean diaper, lots of time in the Moby-and doing less lavishing of attention that I was able to do with my son. Not that I don't interact with DD, but I can't exactly hold her up and make silly faces at her for 20 minutes, either.
I don't like to hear her cry of course, but when she does I know it's for food or a dry diaper-both of which can wait 30 seconds while I finish up with my son or explain what I need to do. Whenever possible, I give him advance notice-that I'll have to feed his sister soon, but after that we can finish building the train tracks. I also do what I can with him while she's eating, and if my hands are full changing her, I make sure to keep talking to him or involve him(like asking him to hand me something). We can easily play a game of I-spy or something when my hands are full.
I make a huge effort not to seem like I am dropping off the radar whenever she needs anything.
But really, unless she's eating or being changed, she MUST be in the Moby or she's hysterical-so it's definitely not easy! That's my only real advice I guess-we're just feeling our way along like everybody else!!