Ziploc size sandwich bags are perfect and a black sharpie marker. I put a bunch of them in an empty plastic grocery bag along with the marker and out the door I go. Seeds go in a zip bag, zip it up, label with the marker and toss in the grocery bag. When I come in the house I put the seeds on the cheapest white paper plates I can find at the store.
Label the plates with the name of the seeds and put them aside to dry for several days before packing them up.
I reuse the baggies too and the plates. Just cross off the name of the plant on the bag and plate and right a new one, until there's no more room to right on.

I use 5 oz. paper cups and a sharpie. I'll harvest, write on the cup the seed, then stack them as I go along. If I need to harvest entire heads, I'll use a 20 oz. plastic cup, or something larger if there's alot or just large heads. I let them dry out for a few days, then put them into ziplock baggies. I used to use paper lunch bags, but this isn't a good idea for tiny seeds; plenty escape out the bottom. For dustlike seeds, I dump the seeds into a coffee filter, fold, then put into the ziplock. I'll then slap a label on the 5 oz. cup and reuse.

I use those foil muffin tins you get at the supermarket, using a different one for every plant. There's different sizes available, you can stack them if you want. And they're reusable :) For the asters and cosmos I use an empty cool whip container since there's so many seeds. When they're dry I just put them into paper envelopes