SlightlyShadySEO wrote an interesting blog post about how you can look at things that no longer work in terms of SEO and use what you've learned to avoid making mistakes that will cause penalties. Here's what he's found from his own observations:
Here's one example about paid links. Beforehand, you could directly solicit for links, use link brokers, and pay for blog posts. SlightlyShadySEO says that there's still doubt about how Google filtered these sites and alludes the penalties to manual reports. If you want to avoid this filter, start doing everything privately from relevant sites.
Other tactics related to link spamming and link farms are also discussed with the was to circumvent the filter. This is a good read.
Forum discussion continues at Sphinn.