One of the main aims of being self sufficient is being able to produce as much produce as you can and being able to produce all the resources needed to do that from your own land. So there is a continuous cycle with very little, if no input form the outside world. So to continue the vegetable garden cycle we need to produce as much of our own seeds as possible. This is not as easy as it sounds as there are many problems that you can come across such as cross pollination from similar crops.
So you need to do a bit of careful planning to prevent similar crops flowering at the same time but there are some crops that don't cross with other plants. Such as peas, beans and tomatoes there are couple of good websites that give lots of information that will help you a lot http://www.howtosaveseeds.com and http://www.realseeds.co.uk/seedsavinginfo.html . Real seeds are a great seed company not only do they only sell open pollinated seeds they are the only seed company that encourages you to save your own seeds. Instead of buying seeds from them every year and they also have some great variety's that you can not buy anywhere else.
In the planning you also have to take into account the fact that some are biennials and some are annuals. So they could take up space in your garden for two years where as the annual variety's grow flower and set seed in the same year. Another alternative is to grow some crops in pots that way you can move them around the garden you can also put them in the greenhouse to protect them at the beginning of the year and help them dry out at the end of the season.
I have left lots of plants in the ground this year so I have lots of my own seed to sow next year just one or two plants left to go to seed. Will produce more than enough seed for your own needs and still have lots of seed left to give or swap with your friends.
One point I have not mentioned yet is the fact that you can not save seeds of F1 hybrids, well you can but you will not get the same plant as you grew. This is due to the way they are bred to start with one other point I should make is that F1 Hybrids are not GMO'S which I hear occasionally. They are bread using natural pollination the only different is they are carefully selected and crossed in controlled conditions. This guarantees you get the same shape, size and colour every time and they also tend to harvest at the same time as well which is not what the home gardener really wants.
Beans and peas are one of the easiest seeds to save they do not cross with other peas and beans so you can grow lots of variety's and get pure seed off them. All you need to do is leave some of the better pods to grow on the plants to mature and dry on the plant.
Dry all your seeds well after harvesting them to make sure they are completely dry then store them in paper bags/envelopes. Making sure you have labelled them with the date of harvest and store them in a dry place and depending on the variety will keep for a couple of years.