Numero uno is of course onpage optimization.
Since you're asking about off page Im assuming your onpage is arredy awesome.
Hands down the most effective strategy is to build your own blog network.
Basically you need to buy expired domains with history.
expireddomains.net is a good place to start.
Then get a couple of $1 hosting on ebay to get unique ips.
Start posting on your sites. Works pretty darn well.
Main idea here is to get the types of backlinks no one else is getting.
Everybody can fire up or buy senuke campaign. These links are free and common.
You cant really rank well with these links. They are only good for pillowing.
With expired domains you get unique links from aged domains with history and backlinks.
It's almost like guest posting. Very effective. Just need to find powerful domains.
If you have a budget you can go to Godaddy auctions and spent a couple of bucks on PR domains.
Of course free links are still important provided they are good quality e.g. links from articles from social media or article sites.
BUt the private network links are the one that will give you significant boost!