Don't #7 - Don't promise what you can't deliver
It may seem really tempting to make promises just to get the deal. However, sales is about relationship building. You can't build a relationship with a client when you start out by failing. You fail when you promise something you can't deliver.
The prospect - now client - feels cheated and lied to. They believe you don't have their best interest in mind - only yours. And why wouldn't they? You told them you could do something that you then couldn't.
Please know this - the client thinks they are buying ALL of what you told them. When some of it turns out to be unavailable, they feel cheated. You won't get more business out of them and you won't get referrals out of them.
So really, what was the point of 'selling' them in the first place?
The lesson here is - deliver on your offer ALWAYS.