The problem, as I believe people see it - is the confusion within the asteroid field, and the perception of the asteroids - be it small ones to collect or large ones 20k off that look almost the same!
Scan them, guys. If you are not sure, of course it will take a while, but you are always able to see them targetted! You cannot click on them on the map, but you ARE ABLE to use the TRACKING TARGET option, and the targetted asteroid chunks will be selected. It's a matter of simply pressing the target's detail button, and checking whether it's collectable.
Before you know it, you'll be piloting your buster (in my case Persius) escort through the asteroid debree and pinpointing the small asteroids and the slightly larger chunks that need to still be shot into smaller pieces.
Do not be afraid to approach this systematically! These asteroids break appart into even chunks in around 6 sectors, and they are quite spread out. Handle one chunk at a time, as those chunks break into some 4 quadrants, and those into even smaller ones. Then you end up with Asteroid debree fields. Fly your small buster close up through the fields, and visually - not by colour, but by simplicity, target the asteroids that seem slightly larger than the rest and shoot them up!
Do not feel afraid to back up and take an overview of your current sector, because it is the only way to beat that 'perception' of your eyes, and maintain a good 'eye' on what's still left to be broken up and what asteroids are ready to collect. Once they're all relatively the same size - call in your freighter to "Follow me" and leave the fighter pointing towards the debree field (This'll help you get used to managing pilotting within 3d and avoid that confusion of which field you were working on).
Slowly, and I mean slowly and straight ahead, guide your freighter at each individial asteroid as if it were your last - and make no sudden turns! Asteroid debree fields make it a nightmare to see, and a nightmare to avoid collisions, unless you are very careful. Be aware of your surroundings, and if you aren't sure whether you can turn down, DON'T - simply back away the path you came, and then do so safely.
Eventually you will be able to tell which asteroids are collectable, because they are the same size when completely broken. And collecting an asteroid field of the same size, you'll get used to the pattern. Get out there, miner!

One huge problem with mobile mining is the time it takes to get used to it. However, from the developers side - leaving the "Open Bay Doors" command on a 7 second pause everytime you collect an asteroid is a HUGE oversight. It's simply ridiculous to have to go through a field of some 30 asteroids and manually open the door every 7 seconds. Had the doorbay been open for an indefinate duration, I would have the whole asteroid plucked within 20 minutes, instead of only 89 Silicon Waffers.

Cheers!