I would not recommend to use directories most are considered as linkfarms so their value is little or maybe even nothing. The best option is producing quality content which is so good that other websites will backlink to it. This is of course the hard option, for simple backlinkbuilding I would suggest to do some value blog commenting and forum posting. If the owner of a blog picks your website up then their is a chance he or she will do a backlink same with forum posting. Social sites are also valuable you must only target the right audience otherwise you get a enormous bounce-rate.