To trim asymmetric spinnakers, concentrate on the luff of the sail. Ease the sheet until the sail starts to roll over (approximately six inches) and then gently sheet in to roll it back.
By repeating this procedure constantly on the downwind leg the spinnaker will always be trimmed at the optimum angle to the wind. Communication between crew and helm is important.
With the crew constantly looking at and trimming the spinnaker, the helm controls the balance by steering. If you are overpowered and start to heel, simply bear away. Likewise, if you are in a lull you can increase power by steering higher. Each different design of dinghy will have an optimum angle for the conditions and this is where experience of a particular design pays.