An on-going challenge your business probably faces is ensuring clients continue using your product or service.

In other words, you want repeat business.

There are few businesses without any competition, so there's a constant struggle to keep existing clients while bringing new clients on-board.

However, many businesses miss a perfect opportunity to sell to their most qualified buyers: current clients.

Around the world, millions upon millions of dollars are spent on brochures, newspaper ads, sales letters and a host of other marketing methods that may not be read, seen or heard.

But what's the one thing from your company that every client reads? It's your invoice.

Unless you have deadbeat clients who don't bother looking at their invoice, each client has to pay at least some attention to it. That's why it's one of the easiest opportunities to make another sale or drum up new prospects.

For example, my invoices usually highlight some other service I offer, or remind the client that I'd appreciate being mentioned to their contacts who may need my writing or design services.

And how much did I spend on this marketing opportunity? Nothing! It doesn't negatively impact my marketing budget.

In some cases, the person reading or paying the invoice is not the decision maker. That's ok. Remember, it doesn't cost you anything to make the attempt, so you're not losing anything if the attempt fails.

In the best-case scenario, you're going to alert the client to one of your services that he or she might not have been aware of.

Or it might jog the person's memory and they'll think of someone who can benefit from your product.

Don't overdo it. The invoice is not the place for an essay. Keep the copy short. I'd suggest no more than two sentences. Save the full-blown sales pitch for a full-blown sales pitch.

And keep the design of the message simple. At most, use a highlighting marker. Another option is to use an asterisk or two to help the message stand out.

Your invoice is a free pass that gets by all the defenses people put up to avoid sales pitches.

Are you wasting this golden opportunity?