We've made a big effort this year to make seed saving a regular part of our gardening efforts. I've tried to minimize inputs (and thus also cost) to have the garden be as self-sustaining as possible. Seed saving is one strategy we've implemented to "close the loop." It's pretty simple and easy and altogether pretty fun.
For starters... we've started letting at least a couple of each type of plant go to seed. So... we have blooming lettuce, cabbage, radish, beets, etc. that form seed puffs or pods that we later collect and crack open to get the seeds out. We are also sure to save a few pea pods or bean pods to be able to replant the peas/beans the next year.
For any of these seeds, we let them dry out completely before collecting them and putting them in envelopes.
For envelopes... we've started saving some of our "junk mail" envelopes by tearing off one of the short ends, taking out the contents, and reusing them as seed envelopes. (This is also good because a lot of curb-side recycling programs won't take your junk mail envelopes unless they are opened up). Then just label them with the name of the plant and the year.
Now, all you need are the seeds. So far... I've saved easy things like the little onions off of the tops of Grandma's Egyptian Walking Onions...
Next year, when it comes time to plant them, we can put them in water to see which seeds are viable. Those that sink are generally viable, and those that float generally are not.
Now, to us, it's like a whole new part of the harvest!