For your Delta rental question, please try posting the same question to the Delta Skymiles forum. You might get better responses. I'm sorry I don't know anything about the Delta rental program. Where I have racked up the most Delta miles is through getting the American Express Delta Skymiles credit card and mileage runs. For any of the mileage-earning cards, usually there is a huge singup bonus that's near or equivalent to a free ticket just for getting the card.

Here's a link to a FTer's own personal site that has a lot of helpful tips: http://www.freefrequentflyermiles.com/index.htm