Collect only good quality seed. Pick seed heads when fully ripe and seed has hardened off, or is very close to hardening off. Pick when there is no moisture on the seed/seed heads eg. no dew, rain etc. Hang or spread out in dry warm place out of sun to complete any drying. Remove debris. Pack only perfectly dry, well formed seed. Label and date. Store in cool place away from light. Some annuals can benefit being stored in the fridge and brought out when it is their usual time to plant. It is preferable to only collect non-hybrid seed. Hybrid seeds are not always true to the parent plant. Also if you grow different varieties of say lettuce, basils, at the same time, you make get "cross" seeds. These can be interesting but will not necessarily be as good as the originals.. Tomatoes, beans, lettuce, pumpkin, cucumber, melon, zucchini, chives, pea, coriander and rocket seeds are well worth saving but other things such as carrot, cabbage, cauliflower, radish can take a while to mature and take up a lot of room in your garden.