Seed saving tips:

Ensuring you get the variety you want….
Two simple rules: only keep the best and prevent cross-pollination. Only keeping the best is straight forward, you simply remove those that look less healthy. Preventing cross-pollination is a bit more involved. For inbreeding varieties, for example peas and French beans, its not a problem as they rarely cross with other varieties. But for outbreeders, such as brassicas, runner beans, alliums and beetroots, you need to either grow just one variety at a time, or find a way to prevent cross pollination with cages or barriers. In all cases save seed only from open-pollinated varieties and not F1 hybrids.

Ensuring you seed lasts a long time….
The two factors that reduce the life of seed are moisture and warmth. Moisture is probably the most important. A dry seed will last much longer at room temperature than a moist seed in the fridge. Dry the seed in a well aired, shady spot, not in full sun. A cool shed is fine for storage, but a fridge or freezer is even better. If the seed has been stored at very cold temperatures, then let it adjust to the ambient temperature for a couple of days before sowing.

For more information, try the Seed Saving Guidelines from the Heritage Seed Library.