Running is undoubtedly the most difficult and tedious point of sailing in a Moth. You should sit quite far forward to reduce the wetted surface area, but not so far forward that you are crouching directly behind the bulkhead. The boat must be kept upright, which can prove difficult when you are going very square to the wind, as you will have no weight to balance against. The kicker should be slack enough so that the top battens are roughly parallel to the boom. The outhaul and cunningham should also be let off as much as possible to give the sail maximum fullness.
Running very square in light winds is easier to achieve in a boat with a bolt-rope sail as you will have softer battens in than a pocket luff sail. One of the inherent traits of pocket luff sails is that they use quite stiff battens in all conditions. This is why you will find that the boom will tend to come in towards the centreline of the boat when conditions are very light. The only solutions to this are to let more kicker off or to use a piece of elastic around the boom and forestay to force the boom to relax.
Sit right at the back perched somewhere around the gunnell/wing join and lock your feet out so that you can react to the gusts lulls and waves. If it gets seriously windy and you are having trouble keeping control over sheeting the main can help as it reduces the downward pressure that is trying to pitchpole the boat.