"The man who skips on copy to save money is like the man who stops the clock to save time."
In all types of marketing, copywriting is the thing that tends to get overlooked. People think that, if they have a great design and hook, copy isn't a big deal. Well, they're wrong, very wrong. The design concept may be the equivalent of your advance troops, but it is the copy that will draw the reader in and seal the deal.
Writing is important because it's the part of the mailer that conveys your key messages. For casinos, sports betting and lottery products especially, some explanation may be needed for both the product and the promotion. It's your chance to draw the reader in and persuade them to respond, so don't rush it or cut corners.
Copy should be direct and relevant; long enough to get your key messages across, but not so long that you lose the attention of the reader. Requirements will differ depending on the campaign, but aim for no more than five sentences as a rule. If you need more than that to explain the promotion, you either need to condense it or your concept may be overcomplicated.
Most readers will only scan your document at first, so choose keywords that are relevant and easy to understand. Use these in much the same way as you would when optimising a website for SEO, but don't repeat them too many times in one paragraph or sentence.
Another consideration is style. The way you write needs to match the brand you're promoting and the campaign itself. For example, if you read the copy used by Foxy Bingo, you will find it is much softer and less formal than the tone adopted by Ritz Club London. The copy always fits the brand.
The simplest thing to remember, as Nev from the BBC's Call Centre programme might say, is that "Good copy sells!" If you really want to do it yourself, there are some useful tips here, but give serious thought to hiring a professional who can bring your concept to life in words.