Now is not the time to rest on your laurels when the first lettuces start to show, but to sow the next row. This year I have been trying wine boxes, which seem to have the advantage of providing a barrier to slugs. I'm not sure if it is the rough wood, or wether they just haven't spotted them yet, but so far so good.
Another invaluable option for bringing seedlings on are paper pots made by a paper potter. They too seem to provide a better barrier for slugs than seed trays.