Respected regulator
Every online casino is operated officially from some country (or territory with a special statute - jurisdiction). To make it possible to operate an online casino from a specific country, this country has to have friendly legislation towards online gambling. Licences are issued either by the state that wants to regulate and tax the online gambling of its own citizens (UK, Belgium, Romania, etc.), or by jurisdictions that want to allow casinos to do international business (Malta, Curaçao, Gibraltar, etc.).
If a casino stubbornly refuses to pay you your legitimate winnings, the only possibility that remains is to turn to the regulator who issued the license to the casino. Only then will it show how important it was to choose a casino according to who issued their licence. A good regulator should always stand independently on the side of fair gambling. He should fairly assess and investigate every official complaint. In case of a serious violation of the rules by a casino, it may accede to revoke their licence.
In practice, with some regulators you won´t even be able to access a contact form in order to submit a complaint (Costa Rica, Panama, Anjouan, Seychelles). In this case, against the willfulness of a casino, you don't have any chance.
With some regulators you might have a chance of obtaining justice, however, their approach will generally be lax. Here we can include Curaçao and Gibraltar.
On the other side of the spectrum, stand respected licencing authorities like Malta, Alderney and Isle of Man. In case of a conflict with a casino licensed by some of these authorities, you have a pretty decent chance of obtaining justice.