>Sew two weeks apart
I do this
>Let a few plants grow to maturity and save seeds
I also do this. Then I hang them under an umbrella and let them dry. Once dry I save some and then I spread a bunch all over my garden in the fall so when spring hits I have lettuce right away. It grows so easily I often have to mow lettuce on my clover lawn.
At the end of the day though, lettuce is sort of a shitty plant to grow, it's just water, I grow a lot more kale, chard and cabbage for my greens. Kale and chard are stupid easy but a nice cabbage grown slowly in a shady bed is so damn good.