One of the most common mistakes that the amateur Junker makes is to store too many things. They end up a waste of effort and eventually end up back in the garbage. The secret is to know "when" enough is enough! Keep it straight that you can only benefit from your efforts if somebody buys the stuff!
Also, don't spend so much time repairing something if you can only get so much back for it. You can't ask more than what the customers are willing to pay!
And when picking stuff up, verbalize yourself around the owner if they are present... it helps to relax them and don't be uptight about asking them if they have more to discard inside. Be Friendly... it'll pay off!