Lettuce, celery, parsnip, rocket, carrots, leeks, onions and radicchio: allow plants to set seed (tall stems of flowers will eventually appear), pick once the seed heads are brown and crisp sounding. Hang upside down in plastic bags for a week or so, rustle the remaining seeds out of the stems and store.
Cucumbers, tomatoes, zucchini, chillies and capsicums: scoop out flesh and wash through a sieve. Lay the seeds on a kitchen towel and place somewhere warm and dry for a week or two before storing. Be careful with chilli seeds as they can sting, so avoid touching your eyes after touching the seeds. Once completely dry, cut the kitchen towel into small pieces and store seeds on the towel in an envelope.