No, it's against forum rules. You can't create threads solely for the purpose of advertising a referral link.
However, if someone asks "Good gambling sites?" or a similar topic, you can reply with a referral link, provided no one in the thread has already mentioned the site.
I think even replying to someones reply with your referral link not allowed but without referral link that is fine. I can suggest that put your referral link in your signature or your personal text or you can create your own blogging site and put all your referrals there and post your blog here in you want to get a lot of referrals.
Nope, it is allowed as stated by hilariousandco, an official bitcointalk moderator. It's good etiquette to label referral links as such, but even if you don't do that, it's not against the rules.
Hey. Referral links are actually allowed in certain threads, for instance if a person asks where they can get free bitcoins or what site should they invest in etc then it's ok to post them, =snip=
Like others have said, PM'ing people is also an option provided it is not overused, however I personally find this annoying and like the above method. They are both allowed on the forum though.