subdomain link building

Most Search engine view subdomains as separate sites.
If you have a web site, your hosting company would probably allow you to create a number of subdomains. Dont waste this resource, build some pages in each subdomain and link back to your other subdomains. Well, I would say dont make it appear on every sub, it would looks like reciporal links. Try some triangle pattern. ie.
A -> B -> C - > A

Backlinks is time consuming but it is proved to be critical in SERP strategy. Spend some minutes everyday to work on it and search engine will reward you.