Expired or For-Sale Domain Best Practices

Can anybody help with a best practices explanation for what to do when it comes to purchasing a domain that may benefit your website?

-What are you looking for in an expired or for-sale domain?

-What do you do once you acquire it? Dyou redirect it to your site, build content on it and then link to your site, etc?

-What to expect in terms of results when your properly purchase and connect the domain.