You may have a new offer coming up and wonder how is the best way to inform your customers of this offer. Ideally you want it to cost as little as possible but get you the most coverage and in turn sales. If your website ranks highly on the search engines for your main phrases, then you may find that new content gets indexed quite quickly and will appear in the search engine results when someone makes a search. But even for the sites that do perform well, it still may not be as quick as you would like or need it to be. Also you are having to rely on people searching for the right thing for your product or service to come up and if it is a new concept you may not get much traffic this way.
Always add new products / offers on to your website, but then you need to direct your customers to this new information and there are a number of ways to do this. To notify existing clients you could do a physical postal mail out or send out an email with links to the new content on the site, but to reach a wider audience, then you could use a pay per click campaign such as Google Adwords. Social Media platforms such as Facebook and Twitter also work really well.