Promotion can be carried out on several levels, and it’s well worth considering which of those levels is going to be the most effective when you have a book to promote.
If you meet someone in a supermarket queue and get chatting, there’s a chance they might ask you what you do for a living. If you tell them you’re a writer, there's then the chance that they might subsequently buy your book. There’s also a chance that you will irritate them quite wholeheartedly if you are too pushy: I read an account a few weeks ago of a vanity-published writer who was thrown out of a restaurant for attempting to sell copies of his book from table to table between courses.
If you give talks, there’s a good chance that you’ll be able to sell copies of your book at the end of the event—just remember that if your talk isn’t particularly informative or amusing then you’re unlikely to sell many copies as your audience will assume that it is as dull as you are.
The very best form of promotion, though, is when you recruit people to act as your books advocate. How? By finding groups of people who have some connection with you, or your book’s theme; by giving them a witty, empathetic, and informative talk and engaging them in conversation in some way, so that they feel involved in the event and connected directly to you; and by making them realise how grateful you offer the opportunity they’ve given you.