Should be similar to HBO - subscription based, or more than that, similar to Tales of Monkey Island (pay upfront, get release over time). In fact, this strategy should be incorporated into the price of the original game (if bought on disc), in that say you bought Fallout 3 when it first came out, then you would get the first year's worth of DLC for free.
In this regard, DLC should be used as an incentive to lure in late adopters, and a reward for those who keep their discs.
Basically, treat DLC as a TV show - make them have an interest to come back every so often.
