We went from one to three when my first was just 2 years old. One thing we did that didn't work well was talk a lot about how exciting it was that he was going to be a "big brother". I think he ended up feeling like his worth was wrapped up in being a big brother, and he was pretty posessive of the babies when they were first born. Once DH and I spent some time talk with him about things that had nothing to do with babies, things got easier.
Related to this, something that I believe has been helpful for our family has been to carefully avoid any type of comparison -- even positive comparisons (e.g., "The baby can't eat cookies, but you can"). Instead, I try to just focus on the child I'm talking to without bringing his siblings into it.
Something else that seemed to work was to properly set our first's expectations as to what a baby can and can't do. Before Cale and Dillon were born, we talked about how babies cry a lot, and that crying is their way to communicate. We even showed Griff some pictures of himself as a baby crying, and he thought they were pretty cool. Griff fussed a lot when he was little, and we worried the twins would, too -- it turned out they hardly fussed at all, which was lucky for us given all of the little kids we had at that point! But, anyway, when Cale and Dillon were born, Griff liked suggesting things they might betrying to tell us when they cried.
I'm sure you've already got it or been recommended it, but I found Siblings without Rivalry
to be a great book -- for some reason it put me at ease that I'd know what was right even with more than one child.
I'm pretty impressed with how well my three boys get along now. Not sure if it's anything we did, though, and often think we're just lucky. But they seem to genuinely enjoy each other. Here's hoping Cynthia's right that adding subsequent children is easy.