After the awesome success of my Angelle tomatoes this year, I've started taking the whole seed-saving thing a bit more seriously. Sure, it's okay to casually dry seeds on a windowsill and use them next year, but if you want your seeds to last longer and get a really good germination rate - and I do - air-dry is not enough. I've been taking the advice of Real Seeds' seedsaving guide (here and here) and drying my saved tomato seeds the proper way.
It's very simple really - just bake dry rice in the oven for 45 minutes or so then put it in a jar (the jar should be about half full, and there should be at least twice as much rice as seed) and put the lid on while it cools. This makes the rice - and the air in the jar, really, really dry. When it's cold, you put your seeds in a breathable pouch made of muslin or old tights, secure it with a rubber band or piece of string, and pop them in the jar on top of the rice for a couple of weeks. The super-dry rice draws all the moisture out of the seeds, and you're left with super-dry seeds that can be sealed in an airtight packet without the risk of suffocating, rotting, or trying to germinate too soon. Super.