I'd say you need a nice looking layout and a catchy headline to get their attention, and useful and/or entartaining content to make them stay. These are the basics, if your site has these then you might want to add in some interaction. If people start to like you and your site they might want to get involved, so why don't you give them the oppurtunity to do so? I always add facebook comments below my articles so my readers can express their thougths. This way it becomes more of a personal experience for them, which is why people who leave comments tend to come back sooner or later
As they say; content is king. While I exercise the brunt of my efforts in creating quality articles, posts, and products to keep people interested by lending information and solving problems, I also feel that the design of a website is equally as important. If your site is very generic, I don't see how you can hope to have a visitor take you seriously. Another thing I do is give viewers a call to action, get or ask them to do something like tweet, like, share, or subscribe for freebies. This tends to keep people coming back for more.
A nice layout that loads fast is a MUST. I also attract my visitors with my content, if the content is good they will keep returning. Of course I run a blog about making money online so almost everyone visits my site to find out how to make money online.
I create relevant content that is being talked about so I could attract more people into my blog. People love talking about current events and I try to relate my posts to those whenever I can. Of course, I try not to force it, since people are also very intuitive to when something is out of place, but I do make an effort to keep things relevant, and I just pepper on some of my personal articles in between popular themed ones.
SEO to attract traffic, of course. Backed with a good content, good product, marketing tactics that work, a professional-quality and prompt customer care service so that the traffic comes back with good feed-backs, and brings their friends along.