The 100% best way to build solid back-links is by not focusing on them at all, and focusing fully on your content. If your content is high quality and provides your audience with valuable information, stronger natural back-links will start to appear. That's not to say some of the other methods don't work, you just have to be a little more creative than before. Your backlinks absolutely MUST be related to your content in one way or another, or you run a high chance of being flagged.
Remember the 2 "R" words : Relevant and Reputable. You could have 1000 backlinks from 1001 small websites with a low PR... and someone else could have 100 backlinks from highly reputable and highly relevant sources with high PRs... and chances are the website with 100 backlinks will outrank the one with 1000
Forum posting has quickly become a poor way to build links as well. If one if posting on forums only to build backlinks, they're wasting their time.
Guest blogging is also becoming pretty iffy to Google, Matt Cutter did a video about this in Nov 2012..
Does Google take action on spammy guest blogging activities? - YouTube
On the other side of the same token, your outbound links should also be related to your content, as search engine crawlers will also check these for relevance. These links should also be limited per page, or you run the risk of being flagged again. this time as a link farm. If it looks like you are just redirecting people to other websites right away, Google will basically take that as you not wanting people on your website.
Ultimately you have to remember that
content is KING, and if your website is made with your audience in mind, as opposed to search engine crawlers, you will always rank much better as well as building stronger backlinks.