Joel Spolsky writes in: "If you ever find yourself implementing a feature simply because it has been promised to a customer, you're drifting over to the land of consultingware and custom development, which is a fine world to operate in if that's what you like, but it just doesn't have the profit potential of off-the-shelf commercial software."
What did we learn from Star Trek II? "The needs of the many outweigh the needs of the few or the one." And given that development is typically 20-30% of the corporate expenditure, it follows that custom work should be billed at 400-500% of its cost, not offered at a discount to win a deal.
Read Joel'sfor some easy tips on prioritizing a product or a release.