2. Run a long term signature campaign and if possible also run a seasonal promotion.
3. Offer higher odds, add many events, add many providers, give higher cashback, promotions etc.
After all it's related to advantage for the gambler and advertisement, creating something new has no guarantee you will success, you need to attract old gamblers to gamble in your site.
Apart from the signature campaigns Solosanz mentioned in the 2nd point, review campaigns are also popular these days, so I think that the return for these actions is also good. Apart from the clear advertising effect these reviews have in the forum, with them you also get direct insight from experienced users' feedback, which is what you seem to be looking for with your last two threads.