Words of advice from Geri Guidetti of The Arc Institute
To sum up storage of seed, cool, dark and dry are the conditions you want. Temperature fluctuations, especially heat, and humidity are seeds' worst enemies. Generally the drier and cooler the better. You are shooting for a moisture content of about 8%. Seed that dry can be safely frozen for very long periods of time with little of no loss of seed viability. I have spoken with seed storage experts at the National Seed Storage Laboratory and was told that seed stored forty years ago under these conditions was highly viable.
A great way to get seed down to such low levels of moisture is to use a desiccant with your seed packets and seal them together in an airtight jar. A Kraft mayo jar, for example, is perfect for a new wide-mouth canning lid and ring. Hellman's and Best Foods mayo jars or standard canning jars will take a regular size canning lid. Add silica gel to the jar, add the seeds, still in their packets, to the jars, and seal. Small seeds will dry down to 8-10% moisture overnight, while large seeds may take several days. You can then recycle the silica gel and process more seeds with it, sealing the dry seeds into a new, dry jar and putting them in the freezer.
Now, if you want to store your seed for a year or two, shoot for the coolest, driest part of your home. Humidity is generally a greater enemy of viability than temperature, but both are important. Most vegetable seeds have a natural longevity of about 3-5 years under these conditions. Onions are less - one year or so. Lettuce, approximately 2 years. Store these in the freezer as above, or grow them out this year and multiply to get fresh seed. If you haven't already bought Suzanne Ashworth's great book, Seed to Seed, I would strongly suggest that you do so. It can be purchased through The Arc Institute for only $23.