Saving Seed

We save our own seed wherever possible. This works really well for most vegetables and flowers. 
Don't try to save seed from F1 hybrids. 
Do choose the best plants to leave to go to seed (always breed from the best stock available!)
Bear in mind that some things may cross fertilise if there are flowering plants available of other varieties to cross with.  This includes wild varieties like wild carrot. Squashes and pumpkins particularly will cross breed between the varieties you are growing - sometimes produces interesting new varieties! Seed saved from plants that are hybrids my not turn out like their parents. You can save your own potatoes seed if you have had no problems with disease in your crop, best to use fresh bought seed every few years.
With things like leeks, carrot, onions, a few plants need to be left in the ground for the next growing season after the crop is harvested to complete their flowering cycle.  I always leave a couple of plants from any of the salad family and brassica family plants for seed. They can cross fertilise if they are in flower at the same time but if you like to experiment this can lead to interesting new plants . 
Store the seeds when totally dry (you can dry them in a warm dry place) in sealed bags or envelopes marked with the date and variety.

useful links:
http://www.seedsave.org/issi/issi_904.html
http://www.rain.org/~sals/miller2.html
http://www.motherearthnews.com/Organic-Gardening/1977-09-01/Save-Your-Own-Garden-grown-Vegetable-Seed.aspx
http://www.farmradio.org/english/radio-scripts/42-1script_en.asp

Our Sowing chart

The quantities are for a group or about 9 people