Keeping your personal financial information safe when spending money online is very important. In this particular case we are talking about spending money at online casinos or other online gambling games such as sports betting, but the points covered are equally valid for any kind of online service provider.
Background Check
You should always do a quick background check on any company that asks for your bank details. If you have initiated the transaction you will presumably have made some research to decide which company to work with but if you have not heard of the company before and came across them online then just do some basic research to see what other customers have to say about them and for other evidence of their legitimate operation.
Onsite Security
Before entering any bank details online you should always ensure that the site you are on is SSL (secure sockets layer) protected. This means that all of the information entered or present on the site or databases is encrypted. Encryption is important to protect you not from the operators of the business you are providing your bank details to but to potential hackers or data scrapers that might try to harvest your data at the point of entry, or subsequently from the site’s database. While this kind of predatory online behaviour is rarer than a few high profile cases might lead you to believe, it does exist and you should both be aware of the possibility and protect yourself against it. The Heartbleed bug shows that even SSL-protected sites are not completely invulnerable to hackers but you are at least doing what you can to protect yourself by only giving personal and financial information on such sites.
Third Party Payment Systems
An even better solution is to where possible use electronic payment providers such as Paypal and Skrill (previously MoneyBookers). Online gambling businesses will almost always support payments made by one or both of those two options, as well as possibly a number of other small providers offering a similar service. With Paypal or an alternative you do not provide the business with any of your bank details. These have been provided to the electronic payment provider when you opened an account. You can usually set up your account to withdraw automatically from your debit card when you process a payment or have a balance with the provider and payments can only be taken from that.
In this case you only provide your Paypal or Skrill details and not any of your bank details, adding an extra layer of security.