Despite the amazing amenities provided by today's cruise ships, there's always something to spend onboard credit on. Maybe it's dinner at a specialty restaurant, drinks, or internet to make your friends and family back home jealous.

If you don't know what onboard credit is, it's free money in your onboard account that will cover discretionary expenses.

The good news is that there's a really easy way to get onboard credit on every cruise you go on, and it works every time. Many hardcore cruisers already know this trick, but some passengers who have gone on dozens of cruises haven't stumbled upon it yet.

As your go-to destination for the best cruise deals on the internet, I feel like it's my duty to share the trick with you. I want to sell you a cruise, but I want to make sure that you get a great value on it and get as much as possible out of your vacation.

Every major cruise line has what's called a Shareholder Benefit Program. If you're a shareholder of the parent company's stock, you get on board credit on every single sailing you go on. There are only three major cruise lines (Norwegian, Royal Caribbean, and Carnival), and they own all of the other ones like Cunard, Costa, and Princess.

You need to buy 100 shares, so with share prices running between $35 and $95, that means that you'd have to invest $3500 to $7000 (of course, prices change frequently so double check this).

The amount of credit you receive is based on how many days you sail for. Each line gives a minimum of $50 for a short cruise and up to $250 for cruises around two weeks or longer.

Only one person per cabin can do this, but you can do it for as many cruises as you want. I personally do it for every single cruise I go on and have received thousands of dollars of free on board credit.

To redeem the credit you e-mail or fax proof of ownership to the cruise line. They almost never write back to confirm that you have the credit, but it always shows up when you get in the ship. If there's a problem, they'll let you know about it.

The one catch is that you can only get one promotional onboard credit per cruise. So if the line is offering $50 OBC and you also do this, it will replace the $50, not add to it (even though they say that, it does sometimes add to it).

You're also supposed to do this a few weeks in advance, but I've done it as late as a few days before the cruise and still gotten the credit.

Technically you could buy 100 shares of the stock, send proof, and then sell it immediately, or right after your cruise. They have no way of knowing how long you hold the stock for. If you cruise a lot like I do, you may want to just hang on to the stock-- after all, cruises are the fastest growing travel segment (which is one of a few reasons I'm in the business).

If you book a cruise through CruiseSheet and need help with this, just shoot me an e-mail. I'll walk you through the whole process and make sure that you get your credit.