G'day all,
I think an important consideration that should be taken into account is windshear on finals or a possible partial (or full) engine failure scenario (as JamesG noted a post or 2 above).
In either of the above situations, it really can save your bacon to be aiming about 250m (or 1/3 into the field, whichever is the lessor) rather than aiming straight at the threshold.
This, of course, depends on where you are operating from.
The school I work for is based at a large radar-controlled airfield with a 2,000m runway. In this location I've found pilots that fly an approach aiming at the threshold, w/flaps 40 (on a 172N) and 2200 RPM, seeming oblivious to the fact that if the engine decides to stop working they're going to lob into the sports field short of the runway.
For a short field landing, the method I'm most familiar with is to aim 1/3 into the field, close the throttle at 50ft (or once obstacles are cleared), fly down and land. It's the method given in the POH's and, whilst it may not deliver *really* short landings, most PPL's are able to get it consistently correct and land safely within the distances given in the POH.
Fly the numbers, read your POH and, above all, keep current.
Cheers,
Grade 3