Interesting article by the economists at the Freakonomics blog about tanking and "being bad to be good." They analyze how many games teams who they determine as excellent (teams winning more than 55 games as 79% of NBA title winners do have done this) have won the years before, even going back 4 years. heres the chart.
What did the 150 "excellent" NBA teams from 1977-78 to 2010-11 do…
…four years before
Number of Teams
Win 55 or more games
Win 50 or more games
Win 40 to 49 games (or "pretty good")
Win 30 to 39 games
Win 20 to 29 games
Win less than 20 games
They go on to give possible explanations for why this might be but if you have time, its an interesting read from a non-sports angle such as ESPN and SI.