seed saving
seed labelling and storage
seed storage life

For maximum germination it's best to use seeds in the first season after they were harvested.  However this is not always possible or practical.  Below is a chart listing some of the most common vegetables and their maximum useful seed storage life.

  Basil 5   New Zealand Spinach 6
  Broad Bean 4   Onion 2
  Broccoli 5   Parsnip 1
  Brussels Sprouts 4   Pea 3
  Cabbage 4   Pumpkin 4
  Carrot 3   Radish 4
  Cauliflower 4   Rocket 2
  Celery 5   Silverbeet 10
  Chinese Cabbage 5   Spinach 5
  Corn 3   Spring Onion 2
  Cucumber 4   Sunflower 5
  Egg Plant 5   Tomato 4
  Leek 3   Turnip 5
  Lettuce 5    

Seeds older than these use by times will still germinate but fertility drops off rapidly beyond these points.  It's best to throw out seeds beyond these use by times. 

Note that these times are for ideal storage conditions.  If seeds are kept in hot or damp conditions they will not last nearly as long.