Lets start off with one of the most basic policies, which really should be needless to say, but don’t click your own ads. Don’t worry, your account wont be disabled for clicking one by accident. But clicking your own ads more than 2-3 times per month will get your account disabled. There’s actually no benefit whatsoever to clicking your own ads. You don’t get paid for those clicks, ever! I’d advise using an AdBlocker to block ads on your own sites, if you are particularly prone to accidental miss-clicks.
We then come to the Incentivized Traffic Policy. This is where AdSense terminates a lot of account holders, and where many people fail to comply with the regulations. You cannot offer any form of incentive for a user to visit your website. This means that not only can you not submit your website to traffic generators, but you cannot engage in any type of CPM advertising, you cannot offer any reward for a user to visit your website, and you cannot post your website link in places where it could be seen as possibly offering a reward. This extends even to buying paid forum posting services. If you’re doing this and running AdSense, be very careful!
Duplicate content is an another big issue for publishers running the AdSense platform. Especially with forums, it can be a hard task ensuring that you are not penalized by Google for having too much duplicate/copied content. AdSense Policies state that websites’ content must be "primarily unique", meaning over 50% of your content will need to be content that only exists on your website. Make sure you don’t copy and paste posts, topics or articles from other sites too often. Also, help your members to understand this also, so that you do not get penalized.
Finally, one of the "silent killer" AdSense termination reasons, is that of having too many advertisements running at the same time. According to Google Adsense’s own policies, you shouldn’t have anymore than 3 of the same type of advertisements, visible on any one page of your website. Excessive advertising gives cause for Google to take the view, that the advertising you are doing for their network isn’t worthwhile, and they can and will shut you down for this. Three advertisement units per page is more than sufficient, and any more than this, you will be running a real risk of having your AdSense account terminated.
Keep ads placed well and do not let them interfere with your website content and you won’t fall foul of any of these guidelines. Don’t exchange clicks and don’t incentivize traffic to your site – these are the usual main reasons why accounts are terminated and people get upset.