One way to preserve the harvest, of course, is to preserve the next one! These are sprouted lettuces. I cut the stalks, place them blossoms-down in paper bags, and then hang them up (closing the bags first) on the walls and rafters of the potting shed. I don’t deal with them, then, until early next spring, when I crush the dried blossoms to release the tiny seeds.
After I bag and hang the seed heads, I chop down the rest of the stalks to about 2-3″ above ground. I cover the bed with a good 2″ of compost, then another 5-6″ or more of grass clippings. The worms appreciate the cover of the clippings and the food in the compost, and the lettuce stalks and roots slowly decompose and aerate the soil at the same time.
All this nonsense takes me about 15 minutes to do. I find I have an endless supply of lettuce seed, though, for a little bit of effort.
Geek notice: Lettuces do cross-pollinate, though the extent to which they do is debatable. This is the second year of these three particular types, and I have grown and harvested them side-by-side the whole time. They still appear to be that which they were originally (namely, Green Oak Leaf, Amish Deer Tongue, and Green Bibb lettuce), so I continue to grow them in the same bed.