Dustin Diaz has put together a nice guide to CSS shorthand. My personal favorite:

element {
  background: #fff url(image.png) no-repeat 20px 100px fixed;
}

Another favorite of mine is shorthand for margin/padding. I always remember the order by thinking of a clockface, but I also recently saw someone recommend “TRouBLe” (top, right, bottom, left). That may work, but I prefer the clockface (especially since that is what I have always used!)