You could tell her how her pregnancy makes her sexier. As her belly grows and all, she might find the idea of being sexy to you rather hard to wrap her head around, and you telling her that will definitely help. Could help her sexy mood, too. ;) When I was pregnant, my husband seemed more disgusted by my growing tummy than anything -- it would have helped me out a ton to hear that he found it attractive.

As far as positions... you kind of have to experiment as she gets further along. Spooning is always a good option. Really, any position that puts you behind her should work, especially once her stomach starts to get in the way. You may be able to pull off missionary, carefully, and putting her on top is another option. We also made good use of the end of the bed... me on the bed (facing him or not) and him standing.