Very seldom. I did buy some Borderlands DLC on a Steam sale (really cheap) aaaaand... I think that's it. I MIGHT buy Skyrim's DLCs, but I'd rather have expansions, like the previous iterations of the franchise.
Other than that, I always pick up games after an year or two they've been released, and, most often than not, the GOTY or Platinum Editions, with all the bells and whistles already included.
I feel the current DLC trend is stimulating studios to release half-baked games, and thus making them strip the core game to a bare minimum, to sell everything else as an "exciting DLC feature". Don't be surprised if you need to buy the "Jump" DLC on some Mario title down the road.
As a customer, I'll oppose that by any means necessary.