1. Deliver just a little more than they expect. Most times when we write a web app for enterprise customers, we try and give them a little bit of extra functionality than they ask for. One example is the user profile settings for their sign on to our web apps - most customers are only bothered with having a username and password, but we often incorporate things like ability to choose avatar images or upload their own images against their profile.
We did this on one education site we developed, and also gave them the ability to choose from about a dozen 'stock' cartoon style avatars if they didn't want to upload their own images. The users were impressed at the handover training session we ran, but I overhead one guy (who was indigenous Australian aboriginal descent) jokingly remark that the stock avatars didn't have a person of indigenous culture represented.
I took note of that, and when I returned to the office, we added a handful of indigenous avatars as well within the hour. Client was happy that we went the extra mile to take their offhand comment seriously and deliver on it.
2. Saying 'No' to 'easy money' projects. We've worked with some of our clients for over 20 years now. Mainly because often when they come to us to add on features to their custom written apps, we often say 'No', along with some valid data to explain why we thing the $$$ sunk into the added feature are of very little benefit.
This has lead to them trusting us a LOT more when we go the other way. Real world case study - we had one client, whom we developed a short term loan application for, ask us to add a Monday morning report with customer mobile numbers so they could do a ring around check for customers who were about to default on their loans.
I said 'Sure, but lets go one better'. I said that along with the report, it wouldn't take much extra work to actually have the system send out an SMS message to all those clients as well, with details on their upcoming defaults, and what they needed to do to fix the issue.
They were delighted and said to go for it. Well, that was two years ago, and it turns out that the SMS messages by themselves have reduced their default rate by an incredible amount, and they are FAR more profitable as a result. Hmm, maybe I should have asked for a percentage of profit increase as my payment! :)