Cordon Tomatoes should be tied in to canes or strings for support and side shoots pinched out from the leaf axils, if left on the plant they will use energy that should be directed to the developing fruit (these shoots are easily rooted to make more plants).
Take out the growing tip of the plants when four flower trusses have formed, this will give more energy to the developing fruit.
Keeping to a good watering and feeding regime for your Tomato Plants is crucial. Use a Liquid Feed High in Potassium to improve fruit quality especially if they are in a grow bag which contains few nutrients. Erratic watering and lack of nutrients can lead to blossom end rot.