water insulator covers that are supposed to prevent freezing and keep the plant alive but I've never used them. If it looks like a possible frost I just cover them up with a bucket for the night.
In my mind, those early days the plant is growing the root system and I don't see so much growth in the leaves. When it warms up the tomato plants take off. I don't know if that is true or not but that is how I think of it.
Through it all, it is completely disheartening to plant something and then have it freeze and you have to start over. The extra couple of weeks aren't going to make a big difference in the long run but I always get antsy once spring hits.