1. Don't talk too much about yourself.
2. Delete posts older than x days.
About 1, well sure thats something, but after posting for a number of years you'd be surprised at what can be found out. Law enforcement have used this to identify prominent hackers in the past, I recall one hacker who as identified from 9 years worth of IRC chat logs, they were able to determine his politicial views among other things and essentially "worked" backwards once they found their suspect.
So lets say they read your posts and find out that John Doe fits the profile of malevolent based on your political views, writing style, information leaks etc. They can work backwards and try to link John Doe to malevolent in order to prove and that John Doe is malevolent. They can do things such as tor timing attacks and using your ISP to do deep packet analysis in order to prove John is malevolent.
Deleting posts is more of security by obscurity, your posts may be stilled indexed by websites such as Google cache and web.archive.org, its also included in forum backups and its possible at least one person is scraping posts on this forum, especially considering we've recently had a number of high profile users posting here such as Ross Ulbricht. Some forums have RSS feeds too and if thats the case it will be in the cache of thousands of users RSS software.
In fact the case against Ross Ulbricht would be much weaker if he had been using an anonymous account to post here.
Google cache doesn't usually go by more than few days back, and the wayback machine doesn't archive everything.
AFAIK, only a few people have forum backups and even less have access to unencrypted backups.
I agree it is possible to find out a lot about people but it's usually because those people aren't good about guarding their personal information and share it too easily with others, often unasked.
Here's an example:
http://antilop.cc/sr/ (scroll down a bit)
If you place a high value on your info you should make a good effort at keeping it private. If you engage in activities that can land you years in prison you should be even more vigilant, maybe also print the relevant pages of the penal code, and hang it above your bed or something
