As for how to pick where to go mining...
Some people deliberately travel 100Ly out of the bubble and then look for a pristine ring.
Personally, I think that's overkill.
Pick a mining site within the bubble, in an inhabited system.
Land at the station, take a look at the station news-feed and check out the local traffic report.
If it says that a hundred Pythons/Kraits/AspXs have been through the system in the last 24h, chances are they've been there to mine in that system, in which case the number of core roids will be reduced.
Some good advice there. I used to travel to a Pristine I knew of inside the bubble, but nowadays I tend to stick to Wolf 25 (my current home system). It may not be the best but it satisfies my mining itch when it crops up.
I'm definitely getting better at spotting the 'roids that aren't worth my time, but there's still plenty that look 'close enough' to fool me. However, I'm doing it because I enjoy it, not because I need the cash as such.