For the past 5.5 years, I've been a part of the product engineering leadership at a couple of organizations. While I'm not sure if these ideas translate to client services, I know that they have all been valuable to me as I work long term building products and brands.

• It's important to periodically reevaluate your tooling and process. Iterating on the process can be just as important as iterating on features. As a team, you are able to take all that you have learned about how you work and try to improve upon it.  It's important to not do this too often though, otherwise, you are spending your time chasing something shiny rather than building something that solves problems. 
• Users over business requirements. Users over short-term wins. Users over everything. If you aren't building for people, your motivations are wrong and you need to rethink what you are doing. Always think about users. If you are discussing working on anything and no one has asked how it benefits users, ask that question. 
• Implementors need to have control over either the schedule or the scope of a project. Giving up both leads to burn out and/or low quality work. Implementors mean everyone actively contributing, and not just the engineers. 
• Design is more important than you think. Design is not done at a specific point. Designers need to be involved in everything. Yes, even the most underhood back-end project. Design thinking is undervalued. I've yet to see it be overvalued.
• Share your work and ideas internally early and often. Even when they are half-baked or you don't think they are very good. 
• It's important to understand that what is a high priority for you, isn't always a high priority for the team. That said, every member of the team should have at least one personal priority accomplished on a regular basis.
• At least once a quarter, you should do a sprint focused on developer experience, refactoring, and bug features. Keeping the code easy to work with shouldn't be forgotten.
• Do things that don't scale, but make them scale if you keep doing them.
• There are a lot of things you can optimize for and deciding what you want to optimize for is a challenge. Sometimes you need to optimize for Time To Launch, others you want to optimize to make it easier to iterate on the UI. Figure out what's important on a project by project basis.
• Don't forget any of Akin's Laws of Spacecraft Design, especially number 1.
• At the end of the day the rules for success are 1) keep the site up 2) faster is better than slower 3) experiment with everything else