"If you accept this price, I'll have a lot more work for you in the future."  Agree to this deal only if the following points are true: You get the commitment to future business in writing.  You know exactly how much future business there will be and exactly how much of it will come your way.   You can afford to do the work or provide the goods at the lower rate. - Michael C. Donaldson, Negotiating for Dummies

We all know the conventional wisdom of just getting your "foot in the door."  It's easy to assume that if you can do a bit of work for a client, they'll see the value in what you offer and certainly come back for more.  But as Michael C. Donaldson reminds us, it doesn't always work out that way, and we should be careful not to get too excited about the potential for future work and suddenly find ourselves actually losing money on the business.  Before ever getting involved in this sort of deal, be certain you take reasonable steps to protect yourself.  At minimum, follow the third bullet carefully: remember that no amount of volume will make up the difference if you're offering your product or service at a loss.