Now a days a lot of off page activities options are available for websites but you wanted to promote betting, gambling and casino websites so I would suggest you to be careful and use the tricks wisely.
Most importantly complete the on-page SEO first and then apply the tricks.
1). Network Community in Social Networks
Create network on social forums, and join many groups where you can create an online image about your website. Share your content frequently with other members to get good engagement for your website. This process is Also known as ORM (online reputation management)
2). Site Blogging
This is the most useful and powerful thing as an off page activity. Create a blog and drive traffic by link exchanges. Enable commenting so that you could get the benefit of UGC(User Generated Content) which is the most preferred content by crawler.
3). Posting on Forums
It’s an old trick but yet it is useful. You can use public forums but keep geo location in your mind while using it. If your website content is targeting a particular audience then make sure you use the right forum. Don’t use any forum you will not get benefit (Sales/Conversions)
4). Search Engine Submission
Submit your website to the most popular search engines like Google, Yahoo, MSN, Altavista, Alexa, Alltheweb, Lycos, Excite, etc., to get listed for free.
5). Photo Sharing
Publish/share your website product pictures and make them public. Let your friends see them and comment on them too, which will help drive traffic towards your website. Do this in major photo sharing websites like Flickr, Picasa, Photo Bucket, Picli, etc.
Note: Always keep in mind that you have to be very careful while implementing these strategies. You should not spam or overdo it. Whatever you do, just plan and execute. Otherwise, you can see negative results.