When you encounter customer stakeholders who are ambivalent or hostile, it's tempting to try to bypass them and get approval from somebody higher up. Not only will this probably backfire (there's a reason they are stakeholders), but you'll end up creating enemies for life.
Better: Engage stakeholders directly, discover why they're ambivalent or hostile, and then work to reach a meeting of the minds.
2. Trash-Talk the Competition
When you're in a competitive situation, it can be tempting to create an "advantage" by pointing out what your competitors do badly. Unfortunately, customers tend to view such negative comments as self-serving, disrespectful, and unprofessional.
Better: Make the case that something your product does well is essential, then provide independent studies comparing your offering with others in that specific area.
3. Give a Sales Pitch
When you're selling, it's tempting to get into "sales mode" and give a sales pitch or presentation explaining why your offering is wonderful. Customers, however, get glassy-eyed and disconnected whenever they hear a sales pitch. (Don't you, when you're on the receiving end?)
Better: Unless you're presenting to a large group, have a give-and-take conversation that identifies problems and opportunities.
4. Cave to Last-Minute Demands
You think you're about to close the deal, but the customer comes back with a surprise demand--and tells you that if you don't agree, the deal is off. It's a huge temptation to give in, simply to close the deal. If you do, though, the customer will rightly conclude that you weren't offering the best deal to start with and therefore aren't trustworthy.
Better: Frame the parameters of the deal fairly and honestly from the very start. When a last-minute demand appears, simply say: "That won't work for me." The customer backs off.
5. Become Complacent
When you've been working with a customer for a long time, it's tempting to believe that you can put the relationship into "maintenance mode." It's very easy in the business world, however, to mistake customer apathy for customer loyalty. It's usually right when you're convinced the relationship is "solid" that the customer is about to switch vendors.
Better: Always assume customers are asking themselves: "What has this guy done for me lately?" Answer the question by doing something that helps the customer become more successful.