No patterns! Which means...
- don't use the same few / several anchors
- don't have only DF links (although, lately I have seen sites with 98% DF links ranking, so I'm not sure that this rule matters as much, but if you want to be safe, mix and vary as much as you can!)
- don't link to the same page while all the other pages on your website don't have a single link. Also in this category, don't link only to pages that target commercial keywords, try to link to info content most of the time (you might still get away with linking to commercial content mostly, but whether or not you'll be penalized for this depends on other factors, such as site authority for example. But generally speaking, linking to informational pages most of the time should be preferred)
- don't build all of your links within few days and then stop. Link building is (and should be) a gradually increasing, and ongoing process.
There are probably a few more rules that I'm missing, but these are some of the most important ones. And like I said, the big rule that encompasses them all is NO PATTERNS! Google are prolific at identifying patterns, and patterns are usually human-made things, so if you have patterns google will be able to tell that they're created by humans, therefore they'll be able to tell you're building links, therefore they'll penalize you
