The rule on a slot at Heathrow is that the airline has to use it a certain percentage of the time over a year (or sublet the slot to another airline, which has to use it), or the slot becomes forfeit.
This allows for some amount of seasonal schedule variation by the airlines.
I don't recall the exact percentage, and it's buried somewhere in the ACL (Airport Coordination Limited) website.
The common thing is to lease the slots out to another airline, especially if it's in respect of something like seasonality.
The other options are to increase flying to a different destination, or add a new destination. I recall Qantas operated a feeder flight from Manchester (flown by a Flightline BAe146), to connect in and out of it's long-haul Kangaroo services, in order to preserve a Heathrow slot. (As opposed to the usual thing of pax connecting on a BA shuttle.)
Of course, we could be headed for a point where the industry is so depressed that once scarce resources such as Heathrow slots, become less scarce, however, I doubt that would happen.
However, I wouldn't at all be surprised to see some of the capacity cuts, both frequency changes and down-gauges, to hang on longer than expected because of the effect they will eventually have on yields - i.e. creating a shortage in the market in order to drive the price up.
Mike
