Tomato seeds can stay viable for many years. Just reseal the packet, place in a ziplock sandwich bag and store in the refrigerator crisper. I have had good germination on tomato seeds that were 10+ years old. As you can see, it is also useful to keep track of the number of remaining seeds in the packets from season to season. Without reopening the packet you will have the information at hand to know when to reorder seeds.