I've read about, thought about, daydreamed about maximizing and extending that season. About the only thing I've actually ever done is mix radish and carrot seeds together, the idea being that the radishes germinate and mature much faster than the carrots, and as the radishes are pulled, they automatically space the carrots properly.
This year, though, I'm committed to carrying through on things like succession planting. We have an early crop of lettuces growing in assorted containers (where we can cover them in the event of unexpected cold weather). The greens in the picture are a mesclun mix, but the seeds are somewhat old, so only a few of the varieties are germinating.
We also have some new, single-variety plantings, of Black-seeded Simpson and a red-leaf lettuce, much like Red Sails.