Correct, signature campaign is the best way promoting gambling sites in this forum and you should run for long period of time, as you can see almost all the best gambling sites has a running campaign to continuous promoting their sites even small amount of fee the important you have more participants wearing your signature code.
It is not the best, but it is one of the best ways to promote gambling site. There are some other best ways such as through the forum ads, social campaign, etc. Signature campaign itself is not enough if there are no interesting offers on the gambling site so signature campaign can be something useless/not effective way to promote the site. All things should come from the website first, once the site has some attractive offers then it can be followed by signature and other campaign.
For sure there are a lot of people who could see the signature campaigns here in bitcointalk and that could lead to exposure of your gambling site which is good. I also agree with the offers because if you offered the greatest and making sure that it is legitimate it would be easier to attract more people to play.