Backlinks may harm !

It is not always good to have back links from lot of sites :
1- if so many in short time it is considered spam
2- if many in long time not related to site activity and users, it is considered paid SEO
3- if from directory, mostly useless
4- if from unrelated site, totally useless

it is important to make the back links increase by flow, let your users give you back links.
if you are a blogger add social bookmarking gadgets, or give the link of the post or page in an html code area, as youtube for example...