Luckily I have LOTS of basil in volume and variety, but unless I want to re-purchase them all each year, I have to make a mad dash each end of season (well, only mad because I procrastinate) to get all my cuttings, seeds, and basil preservation in.
The seed issue is only for the sweet basil which I like to use as companion plants around many of my vegetables like the tomatoes every year, so I need that in abundance. I often forget to cut the flowers/racemes of the other basil varieties off, so many of them self seed in my garden, oftentimes better than the sweet basil which I wish would!
As basil grows quite large, I only take about 3-5 cuttings per plant to make for a manageable pot each winter and then let the actual plant itself die down. Well, what is left at least after I harvest all leaves possible to freeze chopped up in jars with a little canola oil for easy scraping out or whole leaves in bags for flavor and visibility in soups (LIKE TOM YUM). (Ok, if that Tom Yum thing gets annoying, please someone tell me, but it's so hard to suppress the enthusiasm for something so great).
Basil Preservation Primer (very simple):
Take your basil cuttings, strip the last 2-3 leaf nodes of their leaves and place in a cup of water in a sunny place. After they form some good roots, pot them up and enjoy!
Frozen -in Jars w/Canola oil:
Strip all the leaves from the basil you wish to preserve. Puree the leaves in a food processor or blender as fine as you like, drizzle just enough canola oil that it doesn't look oily and won't affect the flavor of what you will eventual cook them into. Find an appropriately sized jar and freeze.
Frozen-in cubes (aka Basil pops):
Same as instructions above minus the oil addition. Place the basil puree in ice cube trays that you can leave you basil either in, or pop them all out eventually for storage in a freezer bag and use the trays to make more basil cubes.
(Um, I would have shown you a picture of this, but it really wasn't pretty... lots of brown... if consensus states that they want to see real, pureed Basil pops in the raw I will post it, but yeah... a little icky)
After rinsing leaves, freeze them whole, or on a tray if you wish. I don't care if mine are wrinkled and get all crunched up, as long as they are relatively whole I'm happy. This method is if you like to see your basil. Fragrant of course still, but not like fresh as you can imagine.
But for the most part, your basil is not going to be as pretty as what you can find in a tube at the grocery store because that stuff has color stabilizers and preservatives and etc. If you are fast and your basil is still looking at its peak (which is doubtful about now) you can prob get a reasonably lovely colored puree. Mine for the most part always gets a bit oxidized and brown tinged at the edges, but it's still delicious!