I forget the details, but TOR can leave a trail. And it doesn't necessarily obfuscate your MAC address.
Besides which, you have to be consistent in your use of TOR.
Lets think about this a bit - the authorities won't just check the contents of one e-mail or forum post. They'll cross-check for any posts through that e-mail account or forum account. Can you be sure that ALL of them were through a correctly configured TOR system? Even when you set the account up? Say you log into an anonymous e-mail account while using TOR, then while still logged in you access facebook. Then later, you log out and again access facebook. Bang, you're caught.
Or consider an e-mail sent to that anonymous account which contains a trojan. Good bet it will get past your security software, and be able to obtain your real IP address and start sending details home. Not much you can do about that - some variants of (for instance) TDSS are virtually undetectable and we know the security services have better tools.
So what realistically can you try to do?
First always access the internet via someone elses link e.g. pub/hotel/starbucks/mcdonalds and only ever use that access point once - and hope there is no camera logging you. And pray that someone hasn't intercepted the wireless signal (which is dead easy)
Or you could use a new 3G dongle each time - but you would have to make sure that each is purchased from a different dealer, and each is used once only, and in a different location
Next obfuscate your MAC address (this can be done, but here's not the place)
Use TOR, but it may be better to run through a series of so-called anonymous proxies in a country which is unlikely to be subject to western request for info e.g. Russia or any of the ex-Soviet asian states
Use a series of unrelated e-mail addresses based in Russia or similar. Use each one only once, and make sure the data given at setup does not ID yourself.
Use a computer with an embedded operating system (e.g Android, or Windows CE) and give it a hard reset before every internet session so theres no chance of infection.
Even then theres not a lot you can do to stop the data being read at the far end.
Of course rather than e-mail you could run your correspondence through the PM systems of a large number of web forums such as this, the more the better - but there are obvious risks with that.
The above is just a simplistic view - in reality you can do much more, but this isn't the place to say what. However to remain truly anonymous is hard once someone has identified you as a risk.