I got a lime basil plant for free when I ordered my peppers and other plants from Lazy Ox Farm. At the start of summer, I used the lime basil to make sun tea quite frequently. But as the season went on, the plant bolted pretty early and I kinda stopped tending to it, aside from watering it regularly.
The other day, I noticed many tiny little seedlings sprouting up around the established plant in the container. These weren't weeds, they were baby basils. The plant had not only gone to seed, it had self-seeded.
That's when I got the obvious light bulb. I could save the seeds (the ones that hadn't fallen to the ground) for next season. And, once the sweet basil was ready, I could do the same with that.
Saving basil seeds is very easy. That plant wants to put out a flower. Most gardeners pick off the flowers, which gets tiring, especially if there is a lot of basil. Instead of picking the flowers off, let them be. They're pretty and they attract bees. Keep in mind that there's a give and take. Once you let the basil flower, the leaves lose flavor. If there's plenty of time left in the season, continue the battle against the flowers. But once fall rolls around, it's time for saving seed.
After a length of time, the small petals will fall off and the flower spike will turn brown and dry out. It won't be pretty any more, but it will finally be useful to you. Trim the spikes, then spread them on a paper towel or paper plate. Let them sit for a few days or a week. To remove the mature seeds, shake the spikes or even rub them. A messier option is to put the spikes in a paper bag and give it a good shake. The seeds will collect in the bottom of the bag, but you'll also have to deal with a lot of chaff.
Basil seeds that have reached maturity are tiny and black. Store them in an envelope for the winter, then plant next spring.
One quick note about cross-pollination. Basil does cross-pollinate, and I don't have the scientific data to give percentages or whatever. But if you grew multiple varieties, keep in mind that there's a risk for cross-pollination and unusual basil plants the next year.