Besides the technical measures in the other comments, there are a bunch of other tricks that might be useful. (Please don't take this as advice, but just as a thought experiment! If I were writing a spy novel, these are some things my characters would do.)
Find a homeless person, and ask him/her to register a bank account in exchange for some cash or a meal. Use that account to set up your website anonymously. You'll find that you need to dirten money if running such an operation - reverse laundering or taking "clean" money and putting it untracably into the business. Such a proxy account is a vital ingredient.
Don't rely on Tor alone. It might be completely subverted, you wouldn't know until it's too late. Buy access to a botnet and route your stuff over it.
Have multiple servers. If you have one single server, its easy to trace. Either by brute force: the ISPs disconnect/slow down 50% of customers for a split second, depending on wheter your site went down or not they know in which half you are, repeat until they find you. There are much more sophisticated techniques that don't require active interference. But if you have at least two or three identical servers at different locations, it makes it a lot harder to catch you. Don't forget tamper-proofing your servers.
Have trustworthy accomplices. Generally, the less people you tell what you are doing, the better. But if you have a close circle of people you can really trust, it becomes much easier to pull this off. You can work from multiple locations, give each other alibis, etc..
Build fake personas. Don't just take a pseudonym, but create fake identities. Keep records on their interests, their motivations, what you disclosed about them. The purpose is to throw investigators off. You should be aware of techniques used by them, such as behavioral analysis, stilistic analysis, etc.. Working with accomplices can help alot in creating these fake identities and concealing your own (e.g. writing style).
Go somewhere safe. If possible, move your servers, or even yourself, somewhere where what you are doing is not punishable, or the authorities can be bribed.
This list could go on for ever... I'm not sure how practical many of these ideas are, but one thing is clear, you'll need a certain amount of "criminal energy" to pull this off - no matter whether your intentions are criminal or not. (Disclaimer: I'm too pussy to actually have done any of the above, so it may or may not work :-))