Well, start by chosing the best pieces for the base. It takes some effort to "mix" stability with aesthetics but you have to have a stable base on which to place the rock. If you find that you don't have any pieces that will accomplish that for you, then I (annoyingly, no doubt) suggest that you get some before you go on. But, with 200-pounds, you probably have some suitable rock for that purpose.
As you build up from the bottom, press down on the rocks before you put more on that. This will ensure that you have stable rockwork.
By the way, it's handy to have some epoxy that's reef-safe like the kinds they sell labeled for that or Aquamend that's widely available, so that if you can't get a stable contact and the rock seems to shift as you press down, you can put some of the epoxy between the rock. That really helps and there'll be no shifting when you use that. But, be sure you want the rock the way it is before you use the epoxy. On the other hand, even if you change your mind after using it, it can be undone.
Try to post some pic....and good luck and have a good time with this.