Having learned my lesson about that long ago, I use a list and a couple accordion files to keep track of my seeds. Accordion files are the types that look like small suitcases and have a different pocket for each letter of the alphabet. You can find them in the office section of a department store.
For the list, you can create a spreadsheet if you like, but I just open a blank Word document. I note the year I plan to plant the seeds at the top of the page. Then I type in "annuals" beneath that and scroll down the page a bit to add "perennials."
Whenever I purchase or receive seeds in trade, I enter each one on my list--in alphabetical order under whichever heading is appropriate. (Tropicals and tender perennials are included under annuals.) I use the Latin name, since common names are frequently shared by more than one type of plant.
You can often find the Latin name in italicized print under the bolder common name on the back of a commercial seed packet. For seeds received in trades or from a friend, you'll often have to make do with as much of the Latin name as possible. For example, I'll describe a packet labeled only "blue columbine" as "aquilegia, blue."
Once I have the names entered in my list, I stash the seeds in one accordion file or the other, using the first letter of the Latin name to indicate which pocket they should occupy. I'll often add a sandwich bag to keep all the packets of one type of seed--say, all the helianthus (sunflowers)--together.
I designate one of the accordion files for seeds I plan to sow the following spring. The other file gets the seeds I plan to trade, which can include both those I've harvested from my garden at the end of summer and extras received in trades from previous years. Around the beginning of June, I usually move whatever is left in my "sow" file over into the "trade" file, to give myself a fresh start. Any seeds I receive in trades after that go in my sow file for the following year, unless they are types which need to be planted while fresh.
You should have a list of what's in the trade file somewhere too, but you can keep that list online, at whatever site you do most of your trading. I keep mine divided up by the years the seeds were harvested or received, to let other traders know how old they are.
Try to remember to list and stash your seeds as soon as you receive them. It's always a good idea to open the packets then to verify that the trader has sent you the right types of seed, and that they are dry and bug-free.
Check to make sure that the handwriting on the packets is legible, and correct it if it's not. It's a good idea to note the year on the packet as well, provided that the trader hasn't already done that for you. If he or she was generous in the amounts sent of some varieties, you can also add those types to your trade list.
Sometime before mid-November, I'll try to order any new commercial seeds I want to try and find planting information for each type of seed I have on my list. I insert my own, easy-to-remember code beside each entry. "C, Q, 1/2," would indicate a seed that requires cool conditions to sprout, germinates quickly, and must be planted a half inch deep.
It's best to get this accomplished early, as some seeds require several months of cold treatment in refrigerated damp paper towels, and I prefer to have that out of the way before my first actual sowing in mid-February. For that sowing, I usually concentrate on seeds labeled VP, for "very pokey!"
I use a variety of sources for germination information, including the commercial seed packets themselves, online germination databases, and seed-starting books such as Norman Deno's excellent Seed Germination Theory and Practice. Deno's code for seeds that require cold treatment is usually just 40-70 or 70-40, to indicate three months of cold treatment at 40 degrees followed by warm treatment at 70, or vice versa. For the seeds that require pretreatment, I can keep track on my list of what date I began that treatment, so I'll know when to move them to a warmer or cooler environment.
For other seeds, I just try to note what date I planted each type and how long the seeds took to germinate. I put all the seeds already planted (or in treatment) in bold print, and change that print to italics once the seeds have sprouted. That way I can tell at a glance what still needs to be started--and what isn't coming up.
Be sure to keep your accordion files stored in a dry, rodent-proof, and easily accessible location. Mine go in a wooden trunk in the living room.
Of course this system only works if you remember to enter and file the seeds as they come in. I tend to get most careless in spring and summer, when I have too many other things to do. I still have seeds I took out of the files to plant this spring, which never got re-filed again. September, with its "back to the daily grind" theme, is a good time to get reorganized. It's also a good time to get out there and gather some seeds!