I can only give my experience as a child here. My sister and are are close spaced, a week shy of 2 years apart. We played together a lot, but we fought a lot too. And my mom and dad's expectations that we would "get along great" only made us not want to get a long. I think we kind of resented that we were "expected to". I especially as a child didn't want to be forced to play with or get along with my little sister. a lot of times I just wanted to be alone or play with my friends. It naturally happened though that we would play together and we would have a great time. As we got older, (like high school) we weren't very close, I think we both tried to distance ourselves from each other. When we were younger were treated almost like twins (identical clothing, identical toys in different colors, etc.), and we wanted to establish our own identities. My sister hung out with the "goth" crowd, wore all black and went to concerts and coffee bars on the weekends, whereas I was a cheerleader and dancer, and heavily into extra curriculars. Very much the over-achiever, not much to do with friends because I was so busy with other activities, though I did have tons of people I would talk with an hang out with at school.
But here's the thing, as adults, we're really close now. We get along really well. My mom is amazed actually. We have very similar beliefs, interests, and similar friends. I think my sister is one of the coolest people I know.
That's just my experience though, my husband's has been a little different (but then he has a huge family, and lots of traumatic childhood experiences too). So from my POV, I would say
-don't force of expect that they will get along or become great friends, it can easily backfire. and NEVER force apologies, they won't mean anything.
-teach and encourage kindness, non violent resolution, and sharing, but make sure each child has their "own" toys too. Encourage them to share, but don't force them to, again it can easily backfire. Tell them how nice sharing is, and how it makes people feel good and how they would want someone to share with them, but not anything like "Let your brother/sister use your toy". A child sees the unfairness in this right away. If something is "theirs" the decision to share it should be theirs as well.
-have times when it's just the kids together like during vacation or home, where there's no friends around. It's easy for the older kids especially to ignore the younger kids. When it's just them, they will naturally play together more.
-Respect their individuality, and differences. Just because one kid like ballet, doesn't mean the other one will. They might like softball, or gymnastics, or gardening. And avoid comparisons like "but your sister is so good at math" (I'm so thankful to my parents for doing the latter).
-take time to spend individual "mom time" and "dad time" with each kid.
These are just some observations from my own childhood for the most part, and from reading your posts Leanna, it sounds like you already do most of them anyway.
.... Maya will be 15 months, and David still 3 1/2. I am hoping they will be close. Does anyone have any btdt advice for me?
first of all...I hope you have a smooth delivery! 
It's funny now, but I surely didn't let that happen again. hehe.
