When you are choosing a domain name there are many factors to consider. How easy is it to spell or remember? Does it contain your target keywords? Sometimes hyphens can help you get keywords or phrases that you might not be available otherwise. A good rule for hyphens is to keep them to a minimum. Use two at the most, any more than that makes your URL look like a spam site.
Read this article: Hyphens in your URL
You already know how important your domain name is and, of course, you want to include target keywords in that domain. But when doing this you should be wary of a common domain name mistake. Keep the hyphens to a minimum. The domain www.cat-and-dog-food-recall.com may be available but look at all of the hyphens. These hyphens make your website look like a spam site, even though you may provide very high quality content. It's better to sacrifice a keyword or two than to come off looking like a shill site.
Read this article: User-friendly URLs
I wrote yesterday about how to choose your domain name. So you already have the perfect .com domain containing your primary keyword and you decide to go out and purchase the .net, .org, etc. What do you do now?
One thing you can do is a 301 redirect which will send any new sub-domain visitors to your primary web page. A 301 redirect is fairly simple to implement when compared with other methods and as long as the user is being redirected to relevant content, they probably won't even notice the difference.
Read this article: How to use your sub-domains
When choosing your domain you should really consider placing your primary keyword in your URL. Having your primary keyword in the domain is a powerful way to show up for that keyword in the search engines. This alone will not get you ranked but it can give you a very solid start.