I only did a few hours mining just to try it out, I'd start on the edge of a cluster and orient using some background feature like a nebula, at least you'll know you aren't doubling back on yourself or going around in circles. The local star is good as well, just keep moving in the direction where your facing the lit side of the 'roids.
The rock you are currently mining will sho on your radar so you can't lose track of that one. I just mine in rings rather than clusters as there is a endless supply of rocks. Even then I have ended up at a rock I had already mined, what are the chances!
This needs to be fixed though. There should be upgrades available for retrieval droids, scanning rocks and marking rocks.