Never do this in selling ...

1. Never put your needs ahead of the interests of the customer.
2. Do not volunteer who your competitors are (unless they are worse) (and unless customer asks)
3. Never send anything important to the customer without reading and when you are tired
4. Do not pitch or answer questions unless (a) you know you have a prospect (b) understand his needs. If you do, they will take your information and share it with their existing vendor, who of course "can do the same thing. 
5. Never ever walk in without reading the bio of who you are about to meet with. There are always great opportunities to learn important insights about their passions, skills and often point of view. Also, by checking linked in, you may realize you have friends and/or associates in common - call them and do some due diligence so you can be even more prepared. Its a little thing but can deliver some hugely important results. 
6. Never, assume that your product is the best thing for the customer. 
7. Never stretch the truth, fib, sugarcoat, lie and put yourself in the position where you can be "found out". In this age of social media it is easy. If you lose credibility, you have lost everything.  
8. Be up front with people you are dealing with. I have found that whilst brutal honesty may not win you today's battle, those that meet you will remember you for the future and you will win the war. 
9. Never be angry when a prospect misleads you and doesn't follow through on something they said they would do.
10.  Never 'badmouth' your competition. If you do, you either come across as jealous or vindictive, or both. When pointing out weaknesses in the competition, do so by subtly representing how you might do it a bit "differently" and let the customer draw his/her own conclusions.  When asked directly what you think of a competitor's presentation, be frank but point out all the good points before summarizing the bad - and then do that "with class." 
11. Never assume the customer knows the answer to their problem.  Often, as sales people, we provide excellent customer service by being responsive and promptly handling their requests.   But we forget we've acquired a valid industry experience ofen with a different perspective to the customers.   Yet we're hesitant to question their plans or suggest an alternative solution to help them achieve their outcomes.  Being an above average salesperson means tactfully questioning your customer's plan at the appropriate time. Never assume the customer knows the only right answer. 
12. When I first started out in sales over 25 years ago, I had a sales job where I was learning how to prospect. At that point in my career, I was easily distracted, and didn't tune people out that came to speak with me at my desk while prospecting. 
13. Pay attention and listen to whom you're speaking, and avoid all else.
14. Never tell a customer you are doing them a favor and providing your services gratis.
15. When client request a decrease in price, never give up immediately. That is a sign that he wants our service, so if we do it, we are probably losing money. You always have time to do it later. Just show a little bit more value, and the sale is done.
16. Learn to loose, before you try to learn to win.   Explaining ... If you are not ready to loose something you´ll probably give more than necessary to win sales.
17. Never tell a customer they're doing something the wrong way.   
18. Never presume your value proposition is what the prospect's value perception is.
19. Never EVER wave the arch rival's brand in front of your prospect when you're making a pitch! Here are two examples of what I mean.   When I was an assistant at a media analysis company, without thinking, we sent a pitch to UPS just like we did with all of our pitches.... in a FedEx package!   When I was in marketing at a big 5 consulting firm, we were courting Pepsi. Our CEO showed up to the meeting with a Coca-Cola in his hand.  
20. Never believe that your job is over once the sale is made. I worked in a company that did highly customized services, and in my first big sale I was told: "Let the project manager handle this from now own, your job is to go on to the next sale." The next time I heard from my customer it was because they were furious about a misunderstanding about scheduling. They weren't angry with the project manager, they were angry with me because I didn't even know there was a problem!  Needless to say, I lost the customer and any future sales to them.
21. Never forget that the word ‘NO’ is not a stop sign but rather an acronym for Next Option/Next Opportunity/Next Offer.  Always ask for the reasons behind the "no. By realizing that ‘NO’ is not a stop sign but rather a detour, it’s allowed me to focus more on my Customer’s options instead of my own ego. 
22. Don't say it unless you're sure about it, and don't EVER make promises you have no right to make. Oh, also? He had been fired for arguing politics with customers one time too many!
23.  Never trust a verbal agreement. In today's economy verbal agreements are worthless.  For all you know the person who shook your hand may not be the only decision maker.
24. Never bluff your way through a technical product presentation.
25. Never call your customers customer and try to sell directly. 
26. Never stop asking questions. We often think we know everything we need to know after we have completed the first few phases of a sales cycle. 
27. Make sure you have thought provoking questions for every visit with a prospect and especially a customer. It takes less time and effort to sell more to an existing customer than to convert a prospect to a customer. 
28. Never stop by, drop in, or check on a customer without adding value in each contact.  
29. Never miss an opportunity to ask a customer WHY they chose to buy from you and equally important, never miss an opportunity to circle back to a loss and ask WHY they did not buy from you. 
30. Never argue with your client, no matter how forceful or antagonistic they are in their objections.  
31. Never ever break a promise even if you think the situation has changed to warrant it. 
32. Remember what is important to us may not be important to our customer. 
33. Never show up late to a meeting
34. Never stick to the "script", paying no attention to what the customer says. As I rookie salesman, I accompanied a senior colleague to a meeting. He just barged ahead with what he wanted to say, not taking any notice of the customer.
35. Never assume who your customer is when they walk through the door.  Talk to all.
36. Never argue with a prospect. That will only make him defend his point of view instead of focusing on the issue at hand - Always ask questions to get him where you want him to be.
37. Never get slipshod on the truth. That one fudged fact always seems to be the be one a prospect remembers.
38. Also Jill always check written material 3 times to catch typo's.
39. When a direct question, problem or unfamiliar topic that I don't have enough info about comes up during a conversation, it is a bad idea to guess. The more important the question and answer, the more important it is to respond, "I don't know, let me find out and get back to you." Addressing the customer's concern honestly builds more credibility than faking an answer.
40. Never assume! (anything about your customer) but LISTEN.
41. Never over-extend your stay.  
42. Reschedule if he keeps you waiting more than 10 minutes.
43. Make sure you never ever fail with what you have promised to do within the specified deadline -be it a phone call, visit, offer, proposal or anything that matters to your prospect/customer even though it may seem unimportant to you. Prospects and customers will always look for signs to make them believe you are credible and interested in them well enough. This is one of the most professional ways to show you care about your prospects and customers. 
44. Never hurt a customer's feelings. 
45. I've had people ask me if it was okay to ask questions or if I was really going to serve them--because some other sales person treated them shabbily. Customers are people, not 'prospects' that you can offend at random. 
46. When working with a prospect using a competitor's product, NEVER run that product down. If you do, you just called your prospect stupid. If they're stupid, you better hope they're stupid enough to do business with you. Your attitude needs to be that the competitive product was purchased by the customer with the best information they had at the time of purchase. Your job is to provide your prospect with new information, and a better opportunity. Keeping in mine that the competitive product itself might be superior to yours. Service and pricing might be the issues. The reasons why you might be a better choice now could be many You need to be prepared to find and present those. #2. If all you can do is run down the competition than you don't have much to offer yourself. #3. It's disrespectful, maybe not overtly, but disrespectful never the less. First to your prospect, and also to your competitor. Competitors are not our enemies. They're out there making a living just like us. Anyway, your prospect may personally like the competitor.
47. On the "Never" section ... ever since the advent of cell phones I've always mandated to "Never, Never, Never" take a "loaded" cell phone into any business meeting of importance ... or rather into any situation that requires you to "pay" attention.
48. "Never be late for an appointment. Never. If it will snow, leave earlier. If there is traffic, leave earlier. If you have a busy day, leave time between appointments. Never be late. It is a sign of disrespect, not caring and not having your act together"