I am in awe of another product manager: the Cranky Product Manager. She posts infrequently but many of her posts are brilliant.
One from last year on “how to say no” to customer or sales feature requests caught my attention. Many of the comments on the post were insightful and mirrored my experience with dealing with feature requests. Generally if you can turn the conversation away from a specific feature and toward the business issue the customer is trying to solve, you can usually either (a) point out another way the software can already do what the customer wants, or (b) give a more informed answer about what it will take to make it happen. The trick is getting away from the specific feature, which the customer and the sales guy may have spent some time building up as The Solution to the problem. To do this, Joel Spolsky’s five Whys is helpful.
Being open with your roadmap is important when you can; it shows the customer that you are thoughtful about the features that do go into the product, gets them thinking about the relative importance of the feature, and makes them feel included in the future of the product.