Over the years I have done a large number of direct mail campaigns followed up by telemarketing.  The big question has always been where to buy the list to be mailed, and I have always felt that the quality of the list was much more important than the price I paid for it.  This really is one of those areas where the more you pay the more it's worth.
I had a client who had the most horrible collateral material. He defended it by saying that it had been successful in delivering leads, but he was looking at it from completely the wrong perspective. What he failed to understand was that while it had been effective, almost any collateral material would have harvested low-hanging fruit and he was simply throwing money away by using what he had as a direct mail piece. 
When I tell people that they should never answer the phone if they don’t know who’s on the other end I often get the objection that people have to be available for their customers.  I don’t necessarily agree with this and think that it is often a question of setting expectations in people’s minds as to how available you are.
We subscribe to Contact Monkey to track e mail open rates. They recently did an analysis of 32 million emails sent by their users and categorized the resulting open rates by subject line.