I have been doing this for many years. The best way to do this most years has been non-repeated airline specific promotions. Examples that allowed one to buy as many miles as you wanted at 1 cent each or less include:

500 AA miles certs on Costco/Sams Club Eggo waffles in 2000, Hallmark/Valuemags bonanza/fiasco in 2001 offering transfer to many airlines, 6000 Qualiflyer (SR/SN/TP) or Qantas miles per $60 IF subscription in 2002, Cheap AsiaMiles (CX) via eBay Anything Points points.com partnership in 2003 and 2004, Cheap DL miles via FTD in 2005 and 2007 and ?, 500 AA miles per pack of Swiss Knights Fondue or Cheese in 2006, Shutterfly/Goldpoints 2007 offer netting cheap miles in a wide choice of airlines, Budget 8,000 DL miles per rental in 2008, US Airways winter shopping promo in 2009, BD miles via Wyndham hotels in 2010, and many more I am not thinking of at the moment.

In many of these promotions, people who played big had to pursue/fight to get all their miles. Also, many of these opportunities died once too many people knew about them. It is not in the best interest of the most savvy to make these opportunities easy to find when they come around unintentionally by the parties offering them. Exposure usually results in the quick demise of the promotion/oppurtunity.

Over the last 12 months, US Airways have been the easiest miles to buy. Several transfer miles promotions have allowed the unlimited purchase of US miles at about 1.05 cents each provided you can use friends/family accounts to transfer between. If recent US promo history repeats itself, August will see another one of these promos. Currently, one can outright buy US miles for ~1.4 cents each. Promos like these US ones are great to share as they are intentional. Many others I mentioned above were not and sharing them too widely quickly killed them.

If cheap miles are important enough to you, you can invest the vast amount of time required to find the less publicized opportunities that come around every year. Again, posting the unintentional options in a FT masterthread is not in the best interest of the FT savvy.