Thats a very good question and one that has vexed me considerably since spring weather nowadays is so variable. On the sandy soil I garden on, if its too cold and wet I end up re-sowing, if its dry seeds will not germinate. The weather can change very quickly and optimum conditions may be rare. So last year I knocked together some frames from fence boards 240cm x 12cm. I place the frames across the bed, sow the seeds, cover with old potting compost and water in with a very fine spray. The frames offer protection from drying wind, the compost covering doesn`t blow away and watering is more effective.
Cheers
Pete