Picture used with permission from Woofens (thanks :)
Some of the suggested info to include on your labels are:
Your DG name
Flower type (annual, biennial, perennial, bulb)
Latin Name (if possible)
Color, if applicable
Zone grown in
Light (full/partial sun, full/partial shade, etc.)
How to start: i.e. scarification, stratifcation, after frost transplant, no transplant, direct seed
Year harvested (important for those growing veggie seed, especially heirloom types.)
Some folks also ask for an e-mail address as well.
Some may not need all the information, but it makes it easier for many. It makes it a lot easier to use the Plant files, if you have at least part of this information. ALWAYS either print this out or write legibly large enough to read easily. I would suggest making a template using this list or omething similar , some of the items are standard, thus you don't have to write as much. For those that trade a lot seed, it should help cut down on the time you're having to spend on your labels and info.
Do not make labels so small a magnifying glass needs to be used to read it. Try to put as much info on the label as you can. It will help identify your seeds.
Small seed bags can be purchased at most craft stores, or there are ziploc sandwich bags and "snack" bags that are perfect for larger seed. Or you can make your own seedpackets from these templates, compliments of many of the DGs on here.
Do make sure your seed is completely dry; getting a envelope full of moldy seed is no fun! There are multiple sites which you can google, including lots of "experts" here on DG on the best ways on saving your seed.
Most experienced seed trades will let you know if they have specific requirements they need to do a trade. For example, some ask for a SASE or a SABSE, that means they want you to send them a self addressed stamp envelope or a one with bubble wrap inside and a large size. Some will ask for postage in addition and will tell you how much they need. Some don't ask for anything except that you "pay if forward" to someone else. It's a good idea to establish how many seeds you are trading for. You can find good information in this trading primer. Do use the trades wanted/have and trading lists available at DG.
If the person you are trading with has a limited number, you can adjust your trade accordingly. It is not in good taste to ask to trade "common" seed for some of the rarer, exotic varieties.
If you explain your circumstance, most of those that have that type of seed are more than willing to "work" a deal with you, if not, ah well, it doesn't hurt to ask. They may be able to help you find a place you can purchase the seed or offer alternative ideas. DGers are for the most part a very friendly, helpful group of gardening addicts.