Harvesting & Saving Lettuce Seed

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I have a confession to make.  We didn't harvest all of our lettuce last year.  I had inadvertently allowed some of my lettuce to grow all summer (read: was too lazy and/or busy to harvest all of it), and the plants went to seed.
It's interesting to see the entire life cycle of a vegetable from start to finish.

The lettuce gets bitter when the outside temperatures get hot in summer (you won't want to eat it after this), it sprouts (grows very tall), then flowers (see above picture), and lastly the flowers produce seed.
After my lettuce went to seed, I figured "why not try to save some seed for next year?" .  At the very least, attempting to save the seed would make me feel better for not having eaten the plants when I should have. They would go to some (attempted) use, even if I botched it, right?  So my thinking went.

Here is what lettuce looks like when it goes to seed.  You can harvest the seed anytime after the flowers die back and resemble a dandelion (below).  (The heirloom lettuce varieties shown are freckles romaine, cimarron romaine, and Amish green deer tongue.)

When I harvested the lettuce seed last fall, I just cut off the tops of the plant stems and tossed them into a box (picture below).  I used a separate box for each variety of lettuce.  Then I stacked the boxes in my office and forgot about them for months.  That was the easiest part, forgetting about them.  :-)
I recently decided it was time to get those boxes out of here.  But I needed to get the seed off the stems first, so I could bag it up for spring planting.
I have no idea how other people do this. I was experimenting and trying to find an easy way.  If there are better methods, post 'em below in the comments section to let me know!
Here is what the seed (still on the stems) looked like in the box:

To get the seed off the stems, I started by using a method I had recently read for harvesting wheat.... beating the stems against the side of a container.  So, I tried smacking the stems against the side of the box.  This worked pretty well, and it was nice to get out some stress & winter frustration by smacking something!  :-)

Next, I just crushed the tops with my hands.  This got off the rest of the seeds.

Here is what I was left with.

The seeds as well as a lot of other debris.  Small pieces of stem, pieces of the fluff from the seed (think of a fluffy white dandelion here), and more.  I picked out the larger stem pieces and packed it all into envelopes.  I figure the debris won't hurt the soil, probably will be good for it, and I had no idea how to easily separate it from the seeds.  (Anyone?  Suggestions?)

For my efforts, I was rewarded with 5 bulging envelopes of organic, heirloom lettuce seed to plant this spring.  Nice!

I was able to clean off some of the seed for use in making handmade paper bookmarks with embedded lettuce seed, which I sell in my online store.  The rest of the seed will be planted in my spring garden.
Happy Planting!  Spring is almost here!