If I have a website and I want to target USA audience then what important changes should I make in my SEO strategy?
if you are trying to ranking in Google.com, then it is best you - have you hosting in the US - domain as .us or .com - your US address on the pages (use the address on your include pages, may be on the footer side of the page) - get more backlinks from US hosted sites and .us and .com sites
I would say yes - you need links from location where you want to rank well, or more precisely they have bigger value then links from the UK for instance.
I'm not sure if backlinking from US hosted websites help a lot in getting ranked in USA. There are many factors, but I suggest the most important is to get the right keywords. If you are offering products or services only to USA people then add US, USA, America, etc. to your keywords. People these days tend to do localized search. So if you are offering in some state then it is better to include that in the keyword. Still not sure if the IP address of a website that links to you can help get more traffic from that country.
To promote any site it is important to see that the site is releated to its own country domain and submission for the Off page work are also happening in the same country sites.
Get the country targeted TLD (top level domain) and use a hosting company in that country. Also, in your Analytics or Google Webmaster you can set what country you want your site to be set in. That should just about do it