I've only been using HAML & SASS for around a month and I'm by no means an expert. I've found that using mixins saves me a lot of time. At the top of my stylesheets I lay out bits that I repeat a lot such as:

  =rounded-corners(!amount)
    border-radius=         !amount
    -moz-border-radius=    !amount
    -webkit-border-radius= !amount

And then add them into the UI:

  .curvybox
    +rounded-corners(7px)