Plant a single variety of spinach. Spinach plants pollinate through the wind, meaning, if planted too closely to another variety, the seeds will likely be hybrids of the two varieties.
Remove spinach plants from your garden that aren't as strong or as colorful as you'd like. Because spinach plants pollinate through the wind, removing "weaker" plants ensures that they don't pass on undesirable traits to other seeds in your garden.
Remove spinach plants that bolt, or flower way ahead of the rest of the crop, because they may pass this tendency to skip to the flowering stage early to the rest of your crop.
Ensure that you have sufficient amount of male spinach plants to pollinate the female plants--female plants bear seeds while males bear pollen. If you have an abundance of healthy male plants, feel free to harvest their leaves.
Harvest only the outside leaves for eating of the plants you intend to save seeds from.
Allow the spinach plants that you wish to collect seeds from to flower, and then fully brown and dry out. Pull up the dried spinach stalks.
Thresh the seeds into a container or strip them off by hand--use gloves if your spinach seeds are of the spiky variety.
Place the seeds in a sealed jar and then store them in a cool dry place--out of direct sunlight--until you are ready to plant them.