During my early years in the job pool, I worked as a waiter for a busy restaurant. One of the most valuable and pervasive lessons I took away from that job was "manage the customer or the customer will manage you".
With that in mind, here's a some quick tips that will help you create and maintain healthy and productive relationships with your clients.
Show Some Love
We, as humans, desire love and approval. That includes your client. Learn to give freely some simple things: A sincere smile, a few kind words, an offer to help without regard for recompense – all of these things will go a long way in building a strong relationship with your client (or with anyone, for that matter). And guess what, it feels good! One of the peculiarities of life is that to get you have to give. It just works, so give it a try!
Your Client is not Your Friend
It's often easy to start seeing clients as friends. After all, you spend a lot of time with them, and you work toward common goals which fosters bonding. Ultimately, however, your client hired you to perform a task, so focus on the job at hand. In the end you'll be judged by the quality and results of your work, so set up for success by making your project goals a priority.
Manage Your Client's Expectations
Explain to your client what they can expect over the course of the project. Make sure they understand that the process isn't perfect, and that things can and will go "bump", but in the end their goals will be met and hopefully exceeded. Continually communicate to your client where you are in the process as well as any issues that may arise. This shows the client that you're working hard to complete the job, and has the added benefit of increasing your client's patience.
Quickly Resolve the Needs of Your Client
Try to anticipate your client's needs before they express them. Doing so will impress your client and you stay focused. Additionally, get (and act on) feedback from your client by occassionally asking how they feel things are going. You may find problems you didn't know existed. That's a good thing!
In the restaurant business, I had a 2-bite check back rule: I would check on the customer after a couple of bites to see if they were satisfied with the meal. This simple action solved countless problems before they became real obstacles to the dining experience of the customer. The same will work for your SEO and web design clients. Check back early and often, but avoid pestering.
Educate Your Client
Remember, you're the professional. Gain your client's confidence by establishing this fact up front. You can do this by educating your client on all aspects of the job at hand. Speak about the job in layman's terms, and don't be overly detailed. Help alleviate any of your client's concerns by continuing to educate as the task progresses. All this said, don't be a blowhard or know-it-all (nobody likes a know-it-all). Make sure to patiently answer any questions your client may have. The key word here is "patiently". Ever deal with a customer service person who acts as if you are bothering them with your "silly questions"? Annoying, right? Patience shows respect, and your client will notice!
Sell Some Dessert!
In my business I only sell what I think my client can use. This goes to my company's main mantra: "The fastest way to success is to help others succeed first." Even so, I won't hesitate to offer extra services beyond those that my client signed up for. Remember, if you don't offer it, someone else will! I personally have been replaced as a consultant on more than one occasion by companies offering my client something I was too reticent to suggest, then took over my work as well. Talk about breakup hangovers!
Hopefully you found a couple of things here that you can put in your SEO or web design toolbag. While not specific to these disciplines (or the discipline of waiting tables, for that matter), the advice you're reading has been borne from experience, so you can be sure it works.
Here are some online resources for help with building good agency/client relationships.